niuersitir  of 


J\'o. 
.Division 


Shelf. 
Received 


<4fc   * 


•*/ 


4^1* 


L^A^f 


*..-  • :  *•• 


. 


B  L  L  s  '      Library. 

^OajUP 

NATURAL  PHILOSOPHY; 


FOB  THB 


USE  OF  SCHOOLS,  ACADEMIES,  AND  PRIVATE  STUDENTS: 


B    LATEST    RESULTS    OP    SCIENTIFIC    DISCOVERT    AND    RESEARCH; 
ARRANGED    WITH   SPECIAL    REFERENCE    TO    THE    PRACTICAL 
APPLICATION   OF  PHYSICAL  SCIENCE  TO  THE  ARTS  AND 
THE  EXPERIENCES  OF  EVERY-DAY  LIFE. 


MTH  THREE  HUNDRED  AND  SEVENTY-FIVE  ENGRAVINGS, 


BY 

DAVID   A.    WELLS,    A.M., 

ATTtnOB  OF  "  THE  BOUNCE  OF  COMMON  THINGS,"  EDITOB  OF  THB  " 
"  r'  SCIENTIFIC  DISCOVERY,"  "  KN'OWLEDGB  I»  POWEB,"  ETC. 


FIFTEENTH  EDITION  REVISED. 

NEW    YORK: 

1YISON,  PHINNEY  &  CO,  48  &  50  WALKER  STREET. 
CHICAGO:  S.  C.  GRIGGS  &  CO.,  39  &  41  LAKE  ST.  * 

CINCINNATI ;   MOORE,  WILSTAOH,   KEYS  A   CO.      ST.   LOUIS  :   KEITH  &  WOODS. 
PHILADELPHIA  :  SOWEB,   BAKNES   fc   CO.      DETUOIT  :   K.   BAYJIOND   *  CO. 

SAVANNAH:  i.  M.  COOPER  k  oo. 
1860. 


Entered,  according  to  Act  of  Congress,  in  the  year  1857,  by 

IVISON    &    PHINNEY, 
In  the  Clerk's  Office  of  the  District  Court  for  the  Southern  District  of  New  York. 


MKOTROTYPBD    BT  PRINTED   BT 

THOMAS  B    SMITH,  J.  D.  BEDFORD   &  CO. 

82  &  84  Beekman-street,  N.  T.  115  &  117  Franklin-street. 


PREFACE, 


THE  constant  progress  made  in  every  department  of 
physical  science,  is  a  sufficient  apology  for  the  prepara- 
tion and  publication  of  a  new  elementary  text-book  on 
Natural  Philosophy. 

The  principles  of  physical  science  are  so  intimately 
connected  with  the  arts  and  occupations  of  every-day 
life,  with  our  very  existence  and  continuance  as  sentient 
beings,  that  public  opinion,  at  the  present  time,  impera- 
tively demands  that  the  course  of  instruction  on  this 
subject  should  be  as  full,  thorough,  and  complete  as 
opportunity  and  time  will  permit.  With  this  view,  the 
author  has  endeavored  to  render  the  work,  in  all  its 
arrangements  and  details,  eminently  practical,  and,  at 
the  same  time,  interesting  to  the  student.  The  illustra- 
tions and  examples  have  been  multiplied  to  a  greater 
extent  than  is  usual  in  works  of  like  character,  and  have 
been  derived,  in  most  cases,  from  familiar  and  common 
objects. 

Great  care  has  been  also  taken  to  render  the  work  com-» 
plete  and  accurate,  and  in  full  accordance  with  the  latest 
results  of  scientific  discovery  and  research. 

In  the  arrangement  of  the  subjects  treated  of,  and  in 
the  incorporation  of  questions  with  the  text,  the  most  ap- 
proved methods,  it  is  believed,  have  been  followed.  The 


IV  PREFACE. 

teacher  will  also  observe  that  the  principles  and  import- 
ant propositions  are  presented  in  large  and  prominent 
type,  and  the  observations  and  illustrations  in  smaller 
letters.  The  advantage  of  this  to  the  learner  is  most 
evident. 

HEAT,  which  is  often  considered  as  belonging  more 
especially  to  chemistry,  has  been  discussed  at  length,  and 
the  familiar  application  of  its  principles  in  the  industrial 
arts,  in  warming  and  ventilation,  in  the  production  of 
dew,  etc.,  carefully  explained.  A  full  and  complete 
outline  of  the  subject  of  Meteorology  has  also  been 
given.  On  the  other  hand,  ASTRONOMY,  which  is  often 
included  in  text-books  on  Natural  Philosophy,  has  been 
omitted,  as  rightfully  and  properly  forming  the  subject 
of  a  separate  treatise. 

An  elementary  work  on  physical  science  can  have  little 
claim  to  originality,  except  in  the  arrangement  and  classi- 
fication of  subjects,  and  the  selection  of  illustrations.  In 
this  respect  the  author  makes  no  pretensions,  and  ac- 
knowledges his  indebtedness  to  the  very  superior  French 
treatises  of  Ganot,  Delaunay,  Archarabault,  and  to  the 
writings  of  Miiller,  Arnott,  Lardner,  Brewster,  and  others. 

The  engravings  in  the  present  volume  are  of  a  superior 
character,  and  have  been  prepared,  in  part,  from  new  and 
original  designs. 

NEW  TORE,  August,  1857. 


CONTENTS. 


PAGB 

INTRODUCTION 9 


CHAPTER    I. 

MATTER,  AND  ITS  GENERAL  PROPERTIES. * . .  11 

CHAPTER    II. 

FORCE 21 

CHAPTER    III. 

INTERNAL,  OR  MOLECULAR  FORCES 22 

CHAPTER    IV. 

ATTRACTION  OP  GRAVITATION 30 

SECTION  I. — WEIGHT 32 

".     II. — SPECIFIC  GRAVITY,  OR  WEIGHT 37 

"     III.— CENTER  OF  GRAVITY 45 

"    IV.  — EFFECTS  OF  GRAVITY  AS  DISPLAYED  BY  FALLING  BODIES  63 

CHAPTER    Y. 

MOTION. 62 

SECTION  I. — ACTION  AND  REACTION 66 

"      II.— REFLECTED  MOTION. 71 

"    IH.— COMPOUND  MOTION.  , 72 


VI  CONTENTS. 

CHAPTER    VI. 

PAGE 

APPLICATION  OF  FORCE 87 

SECTION  I. — THE  ELEMENTS  OP  MACHINERY 93 

"      II.— FRICTION 112 

CHAPTEE    VII. 

ON  THE  STRENGTH  OF  MATERIALS  USED  IN  THE  ARTS,  AND  THEIR  APPLI- 
CATION TO  ARCHITECTURAL  PURPOSES 115 

SECTION  I. — ON  THE  STRENGTH  OF  MATERIALS 115 

"      II. — APPLICATION  OF  MATERIALS  TO  STRUCTURAL  PUR- 
POSES   119 

CHAPTER    VIII. 

HYDROSTATICS 123 

SECTION  I. — CAPILLARY  ATTRACTION 142 

CHAPTER    IX. 

HYDRAULICS 148 

CHAPTER    X. 

PNEUMATICS 163 

CHAPTER    XI. 

ACOUSTICS 183 

SECTION  I. — MUSICAL  SOUNDS 194 

"      II. — REFLECTION  OF  SOUND 197 

"    IH. — ORGANS  OF  HEARING,  AND  THE  VOICE 201 

CHAPTER    XII. 

HEAT 205 

SECTION  I. — SOURCES  OF  HEAT 208 

-"      II. — COMMUNICATION  OF  HEAT 216 

11  ,  III.— EFFECTS  OF  HEAT 227 

"    IV. — THE  STEAM-ENGINE 251 

"     V.— WARMING  AND  VENTILATION.  .                         260 


CONTENTS.  Vil 
CHAPTER    XIII. 

PAGK 

METEOROLOGY 266 

SECTION  I. — PHENOMENA  AND  PRODUCTION  OP  DEW 270 

,4<  *\     II. — CLOUDS,  RAIN,  SNOW,  AND  HAIL 273 

"    III.— WINDS 281 

"    IV. — METEORIC  PHENOMENA 288 

"      Y. — POPULAR  OPINIONS  CONCERNING  THE  WEATHER 291 

CHAPTER    XIY. 

LIGHT 292 

SECTION  I. — REFLECTION  OF  LIGHT 301 

"      II. — REFRACTION  OF  LIGHT * 312 

"     III. — THE  ANALYSIS  OF  LIGHT 325 

"     IV. — THE  EYE,  AND  THE  PHENOMENA  OF  VISION. 347 

"      V. — OPTICAL  INSTRUMENTS 360 

CHAPTER    XV. 

ELECTRICITY 369 

SECTION  I. — ATMOSPHERIC  ELECTRICITY 391 

CHAPTER    XVI. 

GALVANISM 398 

CHAPTER    XVII. 

THERMO-ELECTRICITY 416 

CHAPTER    XVIII. 

MAGNETISM 417 

CHAPTER    XIX. 

ELECTRO-MAGNETISM..                                                                     .  429 


NATURAL   PHILOSOPHY, 


INTRODUCTION. 

What  is  Nat-  *•    NATURAL  PHILOSOPHY,  Or  PHYSICS,  is  that 

phy?PhUoso"     department  of  science  which  treats  of  all  those 
phenomena  observed  in  masses  of  matter,  in 
which  there  is  a  sensible  change  of  place. 

2.  CHEMISTRY,  on  the  contrary,  treats  of  all 
chemistry?       those  phenomena   observed  to   take  place  in 

minute  particles,  or  portions  of  matter,  in  which 
there  is  a  change  in  the  character  and  composition  of  the 
matter  itself,  and  not  merely  a  change  of  place. 

3.  A  falling  body,  the  motion  of  our  limbs 
i!mp?esal5  the    or  °^  machinery,  the  flow  of  liquids,  the  occur- 
NaturSenpw-     rence  °^  sound,  the  changes  occasioned  by  the 
losophy?          action  of  heat,  light,  and  electricity,  are  all  ex- 
amples of  phenomena  which  come  under  the 

consideration  of  Natural  Philosophy. 

Strictly  speaking,  we  have  no  right,  in  Natural  Philosophy,  to  conceive  or 
imagine  any  thing,  for  the  truths  of  all  its  laws  and  principles  may  be  proved 
by  direct  observation, — that  is.  by  the  use  of  our  senses.  When  we  conceive, 
reason,  or  imagine  concerning  the  properties  of  matter,  we  have  in  reality 
passed  beyond  the  limits  of  Natural  Philosophy,  and  entered  upon  the  applica- 
tion of  the  laws  of  mind  or  of  mathematics  to  the  principles  of  Natural  Philos- 
ophy. Practically,  however,  no  such  division  of  the  subject  is  ever  made. 

The  truths  and  operations  of  Chemistry,  in  contradistinction  to  the  truths 
and  operations  of  Natural  Philosophy,  can  not  all  be  proved  and  made  evident 
by  direct  observation.  Thus,  when  we  unite  two  pieces  of  machinery,  as  two 
wheels,  or  when  we  lift  a  weight  with  our  hands,  or  move  a  heavy  body  by  a 
lever,  we  are  enabled  to  see  exactly  how  the  different  substances  come  in 

1* 


10  INTRODUCTION. 

contact,  how  they  press  upon  one  another,  and  how  the  power  is  transmitted 
from  one  point  to  another :  these  are  experiments  in  Natural  Philosophy,  in 
which  every  part  of  the  operation  is  clear  to  our  senses.  But  when  we  mix 
alcohol  and  water  together,  or  burn  a  piece  of  coal  in  a  fire,  we  see  merely 
the  result  of  these  processes,  and  our  senses  give  us  no  direct  information  of 
the  manner  in  which  one  particle  of  alcohol  acts  upon  another  particle  of 
water,  or  how  the  oxygen  of  the  air  acts  upon  the  coal.  These  are  experi- 
ments in  Chemistry,  in  which  we  can  not  perceive  every  part  of  the  operation 
by  means  of  our  senses,  but  only  the  results.  Had  there  been  but  one  kind 
of  substance  or  matter  in  the  universe,  the  laws  of  Natural  Philosophy  would 
have  explained  all  the  phenomena  or  changes  which  could  possibly  take 
place ;  and  as  the  character,  or  composition  of  this  one  substance,  could  not  be 
changed  by  the  action  of  any  different  substance  upon  it,  there  could  be  no 
such  department  of  knowledge  as  Chemistry. 

4.  The  term  PHYSICS  is  often  used  instead 
by^ii8  Term     of  the  term  Natural  Philosophy,  both  having 
physics?          the  same  general  meaning  and  signification. 

It  is  also  customary  to  speak  of  "  PHYSICAL 
LAWS,"  "  Physical  Phenomena,"  and  "  Physical  Theories," 
instead  of  saying  the  laws,  phenomena,  and  theories  of 
Natural  Philosophy. 

5.  A  PHYSICAL  LAW  is  the  constant  relation 
physicairLaws    which  exists  between  any  phenomenon  and  its 

cause.  A  PHYSICAL  THEORY  is  an  exposition 
of  all  the  laws  which  relate  to  a  particular  class  of 
phenomena. 

Thus,  when  we  speak  of  the  "theory"  of  heat,  or  of  electricity,  we  have 
reference  to  a  general  consideration  of  the  whole  subject  of  heat,  or  light,  or 
electricity ;  but  when  we  use  the  expression  a  "  law"  of  heat,  of  light,  or  of 
electricity,  we  have  reference  to  a  particular  department  of  the  whole  subject. 


CHAPTER    I. 

MATTER,  AND  ITS  GENERAL  PROPERTIES. 

1..  MATTER  is  the  general  name  which  has 
j^^  gjven  ^o  fa^  substance  which,  under  an 
infinite   variety  of  forms,   affects   our   senses. 
We  apply  the  term  matter  to  every  thing  that  occupies 
space,  or  that  has  length,  breadth,  and  thickness. 
HOW  do  we        2-  ^  *s  onty  through  the  agency  of  our  FIVE 
SiS^ exists1?"     senses  (hearing,  seeing,  smelling,  tasting,  and 
feeling),  that  we  are  enabled  to  know  that  any 
matter  exists.     A  person  deprived  of  all  sensation,  could 
not  be  conscious  that  he  had  any  material  existence. 
what   is   a        3.  A  BODY  is  any  distinct  portion  of  matter 

body?         existing  in  space. 

what  are  the        ^  ^ne  Pr°Perties,  or  the  qualities  of  matter, 
nSer?6"  °f     are  ^e  Pow<3rs  belonging  to  it,  which  are  capa- 
ble of  exciting  in  our  mind  certain  sensations. 

It  is  only  through  the  different  sensations  which  different  substances  ex- 
cite in  our  minds,  or,  in  other  words,  it  is  by  means  of  their  different 
properties,  that  we  are  enabled  to  distinguish  one  form  or  variety  of  matter 
from  another. 

The  forms  and  combinations  of  matter  seen  in  the  animal,  vegetable,  and 
mineral  kingdoms  of  nature,  are  numberless,  yet  they  are  all  composed  of 
a  very  few  simple  substances  or  elements. 

what  is  a  sim-         *>.  By  a   simple    substance   we    mean   one 
pie  substance?    which  has  never  been  derived  from,  or  sepa- 
rated into  any  other  kind  of  matter. 

Gold,  silver,  iron,  oxygen,  and  hydrogen,  are  examples  of  simple  sub- 
stances or  elements,  because  we  are  unable  to  decompose  them,  convert 
them  into,  or  create  them  from,  other  bodies. 

what  is  the  ^.  ^Q  num^er  °f  the  elements  or  simple 
substances  with  which  we  are  at  present  ac- 
quainted, is  sixty-two. 


12  WELLS'S   NATURAL   PHILOSOPHY. 

7.  These    substances    are    not   all   equally 
Howutedd?trib"    distributed   over    the   surface    of   the   earth  : 

most  of  them  are  exceedingly  rare,  and  only 
known  to  chemists.  Some  ten  or  twelve  only  make  up 
the  great  bulk  or  mass  of  all  the  objects  we  see  around 
us. 

All  the  different  forms  and  varieties  of  matter  are  in  some  respects  alike 
— that  is,  they  all  possess  certain  general  properties.  Some  of  these  prop- 
erties are  essential  to  the  very  existence  of  a  body;  others  are  non- 
essential,  or  a  body  may  exist  without  them.  Thus  it  is  essential  to  the 
existence  of  a  body  that  it  should  occupy  a  certain  amount  of  space,  and 
that  no  other  body  should  occupy  the  same  space  at  the  same  time  ;  but  it  is 
not  necessary  for  its  existence  that  it  should  possess  color,  hardness,  elas- 
ticity, malleability,  and  the  like  non-essential  properties. 

8.  The  following  are  the  most  important  of 
most1  import!     the  general  properties  of  matter — MAGNITUDE 

ofroatter?16"      Or    EXTENSION,    IMPENETRABILITY,     DIVISIBIL- 
ITY, POROSITY,  INERTIA,  ATTRACTION,  AND  IN- 
DESTRUCTIBILITY. 

9.  By  MAGNITUDE  we  mean  the  property 
of  occupying  space.     We  can  not  conceive  that 
a  portion  of  matter  should  exist  so  minute  as 

to  have  no  magnitude,  or,  in  other  words,  to  occupy  no 
space. 

The  SURFACES  of  a  body  are  the  external  limits  of  its  magnitude;  the 
8IZE  of  a  body  is  the  quantity  of  space  it  occupies ;  the  AREA  of  a  body 
is  its  quantity,  or  extent  of  surface. 

The  FIGURE  of  a  body  is  its  form  or  shape,  as  expressed  by  its  bound- 
aries or  terminating  extremities.  The  VOLUME  of  a  body  is  the  quantity  of 
space  included  within  its  external  surfaces.  The  figure  and  volume  of  a 
body  are  entirely  independent  of  each  other.  Bodies  having  very  different 
figures  may  have  the  same  volume,  or  bodies  of  the  same  figure  may  have 
very  different  volumes.  Thus  a  globe  may  have  tea  times  the  volume 
of  another  globe  and  yet  have  the  same  figure,  or  a  globe  and  a  cylinder 
may  have  the  same  volume,  that  is,  may  contain  the  same  amount  of  matter 
within  their  surfaces,  but  possess  very  different  figures. 

10.  By    IMPENETRABILITY   we    mean    that 
property  or  quality  of  matter,  which  renders  it 
impossible  for  two  separate  bodies  to  occupy 

the  same  space  at  the  same  time. 


13 

There  are  many  instances  of  apparent  penetration  of  matter,  but  in  all  of 
them  the  particles  of  the  body  which  seem  to  be  penetrated  are  merely 
displaced.  When  a  nail  is  driven  into  a  piece  of  wood,  the  particles  of 
wood  are  not  penetrated,  but  merely  displaced.  If  a  needle  be  plunged  into 
a  vessel  of  water,  all  the  water  which  previously  filled  the  space  into  which 
it  entered,  will  be  displaced,  and  the  level  of  the  water  in  the  vessel  will  rise 
to  the  same  height  as  it  would  have  done,  had  we  added  a  quantity  of  water 
equal  in  volume  to  the  bulk  of  the  needle.  When  we  walk  through  the  at- 
mosphere, we  do  not  penetrate  into  any  of  the  particles  of  which  the  air  is 
composed,  but  we  merely  push  them  aside,  or  displace  them.  If  we  plunge 
an  inverted  tumbler  into  a  vessel  of  water,  the  air  contained  in  it  will  pre- 
vent the  water  from  rising  in  the  glass — and  notwithstanding  the  amount  of 
pressure  we  may  exert  upon  the  tumbler,  it  cannot  be  filled  with  water  until 
the  air  is  removed  from  it. 

11.  By  DIVISIBILITY  we  mean  that  property 
which  matter  possesses  of  being  divided,  or 
Separated  into  parts. 

It  has  until  quite  recently  been  taught  that  matter  was  infinitely  divisible ; 
that  is,  a  body  could  be  separated  into  smaller  and  still  smaller  particles 
without  limit.  So  far  as  our  senses  inform  us,  this  is  true.  So  long  as  we 
can  perceive  the  existence  of  a  portion  of  matter  by  our  sense  of  sight,  of 
feeling,  of  taste,  or  of  smell,  so  long  we  can  continue  to  divide  it.  Beyond 
this  our  senses  give  us  no  information.  But  the  recent  discoveries  and  inves- 
tigations in  chemistry,  have  proved  beyond  a  doubt,  that  all  bodies  are  ulti- 
mately composed  of  exceedingly  minute  particles,  which  can  not  be  subdi- 
vided. 

12.  To  such  an  ultimate  portion  of  matter 
WbAtom? an    as  *s  no  l°noer  separable  into  parts,  we  apply 

the  term  ATOM. 

The  extent  to  which  matter  can  be  divided  and  yet  perceived 
Extent  to  v,        ky  the  senses  is  most  wonderful. 

can  be  dirid-        A  grain  of  musk  has  been  kept  freely  exposed  to  the  air  of 
ed>  a  room,  of  which  the  door  and  windows  were  constantly  kept 

open,  for  a  period  of  two  years,  during  all  which  time  the  air, 
though  constantly  changed,  was  densely  impregnated  with  the  odor  of  musk, 
and  yet  at  the  end  of  that  time  the  particle  was  found  not  to  have  greatly 
diminished  in  weight.  During  all  this  period,  every  particle  of  the  atmos- 
phere which  produced  the  sense  of  odor  must  have  contained  a  certain  quan- 
tity of  musk. 

In  the  manufacture  of  silver-gilt  wire,  used  for  embroidery,  the  amount  of 
gold  employed  to  cover  a  foot  of  wire  does  not  exceed  the  720,000th  part  of 
an  ounce.  The  manufacturers  know  this  to  be  a  fact,  and  regulate  the  price 
of  their  wire  accordingly.  But  if  the  gold  which  covers  one  foot  is  the 
720,000th  part  of  an  ounce,  the  gold  on  an  inch  of  the  same  wire  will  be  only 


14  WELLS'S   NATURAL   PHILOSOPHY. 

the  8,640,000th  part  of  an  ounce.  "We  may  divide  this  inch  into  one  hundred 
pieces,  and  yet  see  each  piece  distinctly  without  the  aid  of  a  microscope :  in 
other  words,  we  see  the  864,000,000th  part  of  an  ounce.  If  we  now  use  a 
microscope,  magnifying  five  hundred  times,  we  may  clearly  distinguish  the 
432,000,000,000th  part  of  an  ounce  of  gold,  each  of  which  parts  will  be  found 
to  have  all  the  characters  and  qualities  which  are  found  in  the  largest  masses 
of  gold. 

Some  years  since,  a  distinguished  English  chemist  made  a  series  of  experi- 
ments to  determine  how  small  a  quantity  of  matter  could  be  rendered  visible 
to  the  eye,  and  by  selecting  a  peculiar  chemical  compound,  small  portions  of 
which  were  easily  discernible,  he  came  to  the  conclusion  that  he  could  dis- 
tinctly see  the  billionth  part  of  a  grain. 

In  order  to  form  some  conception  of  the  extent  of  this  subdivision  of  mat- 
ter, let  us  consider  what  a  billion  is.  "We  may  say  a  billion  is  a  million  of 
millions,  and  represent  it  thus,  1,000,000,000,000  ;  but  the  mind  is  incapable 
of  conceiving  any  such  number.  If  a  person  were  to  count  at  the  rate  of  200 
in  a  minute,  and  work  without  intermission  twelve  hours  in  a  day,  he  would 
take,  to  count  a  billion,  6,944,944  days,  or  more  than  19.000  years.  But  this 
may  be  nothing  to  the  division  of  matter.  TherB  are  living  creatures  so  mi- 
nute, that  a  hundred  millions  of  them  may  be  comprehended  in  the  space  of 
a  cubic  inch.  But  these  creatures,  until  they  are  lost  to  the  sense  of  sight, 
aided  by  the  most  powerful  instruments,  are  seen  to  possess  arrangements 
fitted  for  collecting  their  food,  and  even  capturing  their  prey.  They  are  there- 
fore supplied  with  organs,  and  these  organs  must  consist  of  parts  correspond- 
ing to  those  in  larger  animals,  which  in  turn  must  consist  of  atoms,  or  little 
particles,  if  we  please  so  to  term  them.  In  reckoning  the  size  of  such  atoms, 
we  must  not  speak  of  billions,  but  of  billions  of  billions.  Such  a  number  can 
be  represented  thus,  1,000,000,000,000,000,000,000,000,  but  the  mind  can 
form  no  rational  conception  of  it.* 

13.  We  use  the  term  MOLECULES,  or  PAR- 
SteitaOTpS£  TICLES  of  matter  to  designate  very  small  quan- 
cies  of  Matter?  fi£GS  of  a  substance,  not  meaning,  however,  the 
ultimate  atoms.     A  molecule,  or  particle  of  matter  may 
be  supposed  to  be  formed  of  several  atoms  united  to- 
gether. 

14.  No  two  atoms  of  matter  are  supposed  to 
s?  touch,  or  be  in  actual  contact  with  each  other, 

and  the  openings  or  spaces  which  exist  between  them  are 

called  PORES.     This  property  of  bodies,  according  to  which 

their  atoms  are  thus  separated  by  vacant  places^ 

What^Poros-    we  ^  pOBOSITY. 

*  The  billion  is  here  used  according  to  the  English  notation.— Vise  Webster. 


MATTER,    AND    ITS   GENERAL   PROPERTIES. 


15 


What  is  the 
evidence  of 
the  existence 
of  Pores  in 
all  matter  ? 


FIG.  1.  If  we  suppose  the  atoms  of  matter  to  consist  of 

minute  spheres  or  globes,  it  is  obvious  that  it  will  be 
impossible  for  them  to  come  into  perfect  contact  at 
all  points :  so  that  there  must  be  small  spaces  be- 
tween them,  where  they  do  not  touch  each  other. 
Fig.  1  represents  the  manner  in  which  we  may  im- 
agine a  collection  of  such  atoms  to  be  arranged  to 
form  a  crystal. 

15.  The  reasons  for  believing  that  the 
atoms  or  particles  of  matter  do  not  ac- 
tually touch  each  other,  are,  that  every 
form  of  matter,  so  far  as  we  are  ac- 
quainted with  it,  can  by  pressure  be 
made  to  occupy  a  smaller  space  than  it  origin- 
ally filled.  Therefore,  as  no  two  particles  of 
matter  can  occupy  the  same  space  at  the  same 
time,  the  space,  by  which  the  size  or  volume  of 
a  body  may  be  diminished  by  pressure,  must,  before  such 
diminution  took  place,  have  been  filled  with  openings,  or 
pores.  Again,  all  bodies  expand  or  contract  under  the 
influence  of  heat  and  cold.  Now,  if  the  atoms  were  in  ab- 
solute contact  with  each  other,  no  such  movements  could 
take  place. 

The  porosity  of  bodies  is  sometimes  illustrated  and  explained 
What  is  gen-  "by  reference  to  a  sponge,  which  allows  the  cavities  which  per- 
by*  the  "term  vade  it  to  be  filled  with  water,  or  some  other  fluid.  Such  an 
Pores  ?  illustration  is  not  strictly  correct.  The  cavities  of  a  sponge  are 

not  really  its  pores,  any  more  than  the  cells  of  a  honey-comb 
are  the  pores  of  wax.  In  common  speech,  however,  the  term  pore  is  often 
used  to  designate  those  openings  which  exist  naturally  in  the  substance  of  a 
body,  which  are  sufficiently  large  to  admit  of  the  passage  of  fluids  like  water, 
and  gases  like  air. 

Several  very  important  properties  of  matter  are  dependent  on  porosity ;  or, 
in  other  words,  they  owe  their  existence  to  the  fact,  that  the  particles  of  mat- 
ter do  not  actually  touch  each  other.  The  principal  of  these  are  DENSITY, 
COMPRESSIBILITY,  and  EXPANSIBILITY.  These  properties  of  matter  belong  to 
all  bodies,  but  not  to  all  alike. 

16.  By  DENSITY  we   mean  the   proportion 

whatisDens-     ^ich  exists  between  the  quantity  of  matter 

contained  in  a  body  and  its  magnitude,  or  size. 

Thus,  if  of  two  substances,  one  contains  twice  as  much 


16  WELLS'S   NATURAL   PHILOSOPHY. 

matter  in  a  given  space  as  the  other,  it  is  said  to  be  twice 
as  dense. 

There  is  a  direct  connection  between  the  density  of  a  body  and  its  porosity. 
A  body  will  be  more  or  less  dense,  according  as  its  particles  are  arranged 
closely  together,  or  are  separated  from  each  other ;  and  hence  it  is  clear,  that 
the  greater  the  density  the  less  the  porosity,  and  the  greater  the  porosity  the 
less  the  densit}?-. 

17.  If  the  particles  of  a  body  do  not  touch  each  other,  then,  if  it  is  subjected 
to  pressure,  they  may  be  forced  nearer,  and  made  to  occupy  less  space. 

This  we  find  to  be  the  fact.  All  matter  may  be  compressed.  The  most 
solid  stone,  -when  loaded  with  a  considerable  weight,  is  found  to  be  com- 
pressed. The  foundations  of  buildings,  and  the  columns  which  sustain  great 
weights  in  architecture,  are  proofs  of  this.  Metals,  by  pressure  and  hammer- 
ing, are  made  more  compact  and  dense.  Air,  and  all  gases,  are  susceptible  of 
great  compression.  Water,  and  all  liquids,  are  much  less  easily  compressed 
than  either  solid  or  gaseous  bodies. 

18.  By  COMPRESSIBILITY,  therefore,  we  mean 
that  property  of  matter  in  virtue  of  which  a 
body  allows  its  volume  or  size  to  be  diminished, 

without  diminishing  the  number  of  the  atoms  or  particles 
of  which  it  is  composed. 

19.  Again,  if  the  particles  of  matter  of  which 
a  body  is  composed  do  not  touch  each  other,  it  is 
clear  that  they  may  be  forced  further  apart. 

This  we  find  to  be  the  case  with  all  matter.  Expansibility 
is,  therefore,  that  property  of  matter  in  virtue  of  which  a 
body  allows  its  volume  or  size  to  be  increased,  without  in- 
creasing the  number  of  the  atoms  or  particles  of  which  it 
is  composed. 

All  bodies,  when  submitted  to  the  action  of  heat,  expand,  and 
Illustrations  occupy  a  larger  space  than  before.  To  this  increase  in  dimen- 
bility  ?PM  sions  there  is  no  limit.  "Water,  when  sufficiently  heated,  passes 

into  steam,  and  the  hotter  the  steam  the  greater  the  space  it 
will  occupy.  All  bodies,  if  subjected  to  a  sufficient  degree  of  heat,  will  pass 
from  the  state  of  solids  or  liquids,  into  the  state  of  vapor,  or  gases. 

20.  INERTIA  signifies  the  total  absence  in  a 
Whaertia?In"     ^ody  of  all  power  to  change  its  state.     If  a 

body  is  at  rest,  it  can  not  of  itself  commence 
moving  ;  and  if  a  body  be  in  motion,  it  can  not  of  it- 
self stop,  or  come  to  rest.  The  motion,  or  cessation  of 


IT 

motion  in  a  body,  requires  a  power  to  exist  independent 
of  itself. 

It  is  obvious,  from  the  definition  given,  that  when  a  body  is  once  put  in 
motion,  its  inertia  will  cause  it  to  continue  to  move  until  its  movement  is  de- 
stroyed, or  stopped,  by  some  other  force. 

A  ball  fired  from  a  cannon  would  move  on  forever,  were  it  not  for  the  re- 
sistance or  friction  of  the  air,  and  the  attraction  of  the  earth. 

21.  By  FRICTION,  we  mean  the  resistance 
whationfric"    which  a  moving  body  meets  with  from  the 

surface  on  which  it  moves. 

A  marble  rolled  upon  a  carpet  will  move  but  a  short  distance,  on  account 
of  the  roughness  and  unevenness  of  the  surface.  Its  motion  would  be  con- 
tinued much  longer  on  a  flat  pavement  and  longer  still  on  fine,  smooth  ice. 
If  friction,  the  attraction  of  the  earth,  and  the  resistance  of  the  air,  were  en- 
tirely removed,  the  marble  would  move  on  forever. 

Owing  to  the  property  of  inertia,  or  the  indifference  of  mat- 
What  are  Ex-  ter  to  change  its  state,  we  find  it  difficult,  in  running,  to  stop 
eS1?8  °f  lQ~  ail  afc  once-  Tne  bodv  tends  to  go  on,  even  after  we  have  ex- 
erted the  force  of  our  muscles  to  stop.  We  take  advantage  of 
this  property,  by  running  a  short  distance  when  we  wish  to  leap  over  a  ditch 
or  chasm,  in  order  that  the  tendency  to  move  on,  which  we  acquire  by  run- 
ning, may  help  us  in  the  jump.  Tor  the  same  reason,  a  running-leap  is  al- 
ways longer  than  a  standing  one. 

Many  of  the  most  frightful  railroad  accidents  which  have  happened,  are  due 
to  the  laws  of  inertia.  The  locomotive,  moving  rapidly,  is  suddenly  checked 
by  an  obstruction,  collision,  or  breakage  of  machinery ;  but  the  train  of  cars, 
in  virtue  of  the  velocity  previously  acquired,  continue  to  move,  and  in  conse- 
quence are  driven  into,  or  piled  upon  each  other. 

For  the  same  reasons  the  wheel  of  an  engine  continues  to  pursue  its  course 
for  a  time  after  the  driving  force  has  stopped.  This  property  is  taken  advan- 
tage of  to  regulate  the  motions  of  machinery.  A  large,  heavy  wheel  is  used 
in  connection  with  the  machinery,  called  a  FLY-WHEEL.  This  heavy  wheel, 
when  once  set  in  motion,  revolves  with  great  force,  and  its  inertia  causes  it 
to  move  after  the  force  which  has  been  imparted  to  it  has  ceased  to  act.  A 
water-wheel  or  a  steam-engine  rarely  moves  perfectly  uniformly,  but  as  it  is 
not  easy,  on  the  instant,  either  to  check  or  increase  the  movement  of  the 
heavy  wheel,  its  motion  is  steady,  and  causes  the  machinery  to  which  it  is 
attached  to  work  smoothly  and  without  jerking,  even  if  the  action  of  the  driv- 
ing force  be  less  at  one  moment  than  at  another. 

22.  ATTRACTION  is  that  tendency  which  all 
Action  ?At*    the  particles  of  matter  in  the  universe  have  to 

approach  to  each  other.* 

*  As  Attraction,  in  its  various  forms  and  relations  to  matter,  is  so  comprehensive  and 
Important,  it  is  treated  separately  in  advance. 


18  WELLS'S   NATURAL   PHILOSOPHY. 

The  force  which  holds  the  particles  of  a  stone,  a  piece  of 

What  are  Ex-      wood,  or  metal  together,  the  falling  of  a  body  to  the  earth,  the 
amples  of  At-  ,  ,        ,        , 

traction  ?  tendency  which  a  piece  of  iron  or  steel  has  to  adhere  to  a  mag- 

net, are  all  familiar  examples  of  the  different  forms  of  attraction. 

23.  All  the  researches  and  investigations  of 
modern  science  teach  us,  that  it  is  impossi- 
ble for  any  finite  agent  to  either  create  or  de- 
stroy a  single  particle  of  matter.  The  power  to  create 
and  destroy  matter  belongs  to  the  DEITY  alone.  The 
quantity  of  matter  which  exists,  in  and  upon  the  earth  has 
never  been  diminished  by  the  annihilation  of  a  single 
atom. 

"When  a  body  is  consumed  by  fire,  there  is  no  destruction  of  matter :  it 
has  only  changed  its  form  and  position.  "When  an  animal  or  vegetable  dies 
and  decays,  the  original  form  vanishes,  but  the  particles  of  matter,  of  which  it 
was  once  composed,  have  merely  passed  off  to  form  new  bodies  and  enter  into 
new  combinations. 


PRACTICAL   QUESTIONS  ON   THE   PROPERTIES   OF   MATTER. 

1.  Why  will  water,  or  any  other  liquid,  when  poured  into  a  tunnel  .closely  inserted  into 
the  mouth  of  a  bottle,  run  over  the  sides  of  the  bottle  ? 

Because  the  bottle  is  filled  with  air,  which,  having  no  means  of  escape, 
prevents  the  water  from  entering,  since  no  two  bodies  can  occupy  the  same 
space  at  the  same  time.  Ifj  however,  the  tunnel  be  lifted  from  the  bottle  a 
little,  so  as  to  afford  the  air  an  opportunity  to  escape,  the  water  will  then 
flow  into  the  bottle  in  an  uninterrupted  stream. 

2.  Are  the  pores  of  a  body  entirely  empty,  vacant  spaces  ? 

The  pores  of  a  body  are  often  filled  with  another  substance  of  a  different 
nature.  Thus,  if  the  pores  of  a  body  be  greater  than  the  atoms  of  air,  such  a 
body  being  surrounded  by  the  atmosphere,  the  air  will  enter  and  fill  its  pores. 

3.  When  a  sponge  is  placed  in  water,  that  liquid  appears  to  penetrate  it.    Does  the  water 
really  enter  the  SOLID  particles  of  the  sponge? 

It  does  not ;  it  only  enters  the  pores,  or  vacant  spaces  between  the  par- 
ticles. 

4.  When  we  plunge  the  hand  into  a  mass  of  sand,  do  we  PENETBATE  the  sand  ? 
We  do  not ;  we  only  displace  the  particles. 

5.  Why  do  bubbles  EISE  to  the  surface  when  a  piece  of  sugar,  wood,  or  chalk  is  plunged 
under  water  ? 

Because  the  air  previously  existing  in  the  pores  becomes  displaced  by  the 
water,  and  rises  to  the  surface  as  bubbles. 

6.  What  occasions  the  SNAPPING  of  wood  or  coal  when  laid  upon  the  fire  ? 


MATTER,    AND   ITS   GENERAL   PROPERTIES.  19 

Because  the  air  or  liquid  contained  in  the  pores  becomes  expanded  by  heat, 
and  bursts  the  covering  in  which  it  is  confined. 

7.  Why  does  LIGHT,  POROUS  WOOD,  like  chestnut  or  pine,  make  more  snapping  in  burn- 
ing than  any  OTUEB  kind  ? 

Because  the  pores  are  very  large,  and  contain  more  air  than  wood  of  a  closer 
grain,  like  oak,  etc. 

8.  How  is  water,  or  any  other  liquid,  made  PUBE  by  filtering  through  paper,  cloth,  a 
layer  of  sand,  rock,  etc.  ? 

The  process  of  filtration  depends  on  the  presence  of  pores  in  the  substance 
used  as  a  filter,  of  such  magnitude  as  to  allow  the  particles  of  liquid  to  pass 
freely,  but  not  the  particles  of  the  matter  contained  in  it,  which  we  wish  to 
separate. 

9.  Why  is  not  the  substance  suitable  for  the  filtration  of  ONE  liquid  equally  adapted  for 
the  filtration  of  ALL  liquids  ? 

Because  the  magnitude  of  the  pores  in  different  substances  and  of  the  im- 
purities in  liquids  is  different ;  and  no  substance  can  be  separated  from  a 
liquid  by  filtration,  except  one  whose  particles  are  larger  than  those  of  the 
liquid. 

10.  Gold  and  lead  are  metals  of  great  density ;  their  pores  are  not  visible.     Is  there  any 
PBOOF  of  their  existence  beside  the  fact  that  they  can  be  compressed  ? 

Water  can  be  forced  mechanically  through  a  plate  of  lead  or  gold  without 
rupturing  any  portion  of  the  metal.  Mercury,  or  quicksilver,  confined  in  a 
dish  of  lead  or  gold,  will  soak  through  the  pores,  and  escape  at  the  bottom. 

An  interesting  experiment  was  tried  at  Florence,  Italy,  nearly  two  centu- 
ries ago,  which  furnished  a  striking  illustration  of  the  porosity  of  so  dense  a 
substance  as  gold.  A  hollow  ball  of  this  metal  was  filled  with  water,  and  the 
aperture  exactly  and  firmly  closed.  The  globe  was  then  submitted  to  a  very 
severe  pressure,  by  which  its  figure  was  slightly  changed.  Now,  it  is  proved 
in  geometry,  that  a  globe  has  this  peculiar  property — that  any  change  what- 
ever in  its  figure  necessarily  diminishes  its  volume,  or  capacity.  The  result 
was,  that  the  water  oozed  through  the  pores,  and  covered  the  surface  of  the 
globe,  presenting  the  appearance  of  dew,  or  steam  cooled  by  the  metal  This 
experiment  also  proved  that  the  pores  of  the  gold  are  larger  than  the  element- 
ary particles  of  water,  since  the  latter  are  capable  of  passing  through  them. 

11.  When  a  CABBIAGB  is  in  motion,  drawn  by  HOHSES,  why  is  the  same  exertion  of  power 
in  the  horses  required  to  STOP  IT,  as  would  be  necessary  to  BACK  IT,  if  it  were  at  rest  ? 

Because,  according  to  the  laws  of  inertia,  tho  force  required  to  destroy  mo- 
tion in  one  direction  is  equal  to  that  required  to  produce  as  much  motion  in  the 
opposite  direction. 

12.  If  a  carriage,  railroad-car,  or  boat,  moving  with  speed,  be  suddenly  STOPPED  or  BE- 
TABDKD,  from  any  cause,  why  are  the  passengers,  or  the  baggage  carried,  precipitated 
from  their  places  in  the  BISECTION  OF  THE  MOTION  ? 

Because,  by  reason  of  their  inertia,  they  persevere  in  the  motion  which  they 
shared  in  common  with  the  body  that  transported  them,  and  are  not  deprived 
of  that  motion  by  the  same  cause. 


20  WELLS'S   NATURAL   PHILOSOPHY. 

13.  Why  will  a  PEESON,  leaping  from  a  carriage  in  rapid  motion,  fall  in  the  direction  in 
which  the  carriage  is  moving  at  the  MOMENT  his  feet  meet  the  ground  ? 

Because  his  entire  body,  on  quitting  the  vehicle  and  descending  to  the 
ground,  retains,  by  its  inertia,  the  progressive  motion  which  it  has  in  common 
with  it.  When  his  feet  reach  the  ground,  the}^,  and  they  alone,  will  be  sud- 
denly deprived  of  this  progressive  motion,  by  the  resistance  of  the  earth,  but 
the  remainder  of  his  body  will  retain  it,  and  he  will  fall  as  if  he  were  tripped. 

14.  Why  is  a  man  standing  carelessly  in  the  STEBN  of  a  boat  liable  to  fall  into  the  water 
behind,  when  the  boat  begins  to  move  ? 

Because  his  feet  are  pulled  forward  while  the  inertia  of  his  body  keeps  it  in 
the  same  position,  and,  therefore,  behind  its  support.  For  a  similar  reason, 
when  the  boat  stops,  the  man  is  liable  to  fall  forward. 

15.  When  the  sails  of  a  ship  are  first  spread  to  receive  the  FOECE  or  IMPULSE  of  the  wind, 
why  does  not  the  vessel  acquire  her  full  speed  at  once  ? 

Because  it  requires  a  little  time  for  the  impelling  force  to  overcome  the  in- 
ertia of  the  mass  of  the  ship,  or  its  disposition  to  remain  at  rest. 

16.  Why,  when  the  sails  are  taken  in,  does  the  vessel  continue  to  move  for  a  considerable 
time? 

Because  the  inertia  of  the  mass  is  opposed  to  a  change  of  state,  and  the  ves- 
sel will  continue  to  move  until  the  resistance  of  the  water  overcomes  the  op- 
position. 

17.  Why  do  we  KICK  against  the  door-post  to  SHAKE  the  snow  or  dust  from  our  BIIOEB  ? 
The  forward  motion  of  the  foot  is  arrested  by  the  impact  against  the  post ; 

but  this  is  not  the  case  with  respect  to  the  particles  of  dust  or  snow  which 
are  not  attached  to  the  foot,  and  are  free  to  move.  According  to  the  laws 
of  inertia,  they  tend  to  persevere  in  the  direction  of  the  original  motion,  and 
when  the  foot  stops,  they  move  on,  or  fly  off. 

18.  Why  do  we  BEAT  a  coat  or  carpet  to  EXPEL  the  dust? 

The  cause  which  arrests  the  motion  imparted  to  the  coat  or  carpet  by  the 
blow  does  not  arrest  the  particles  of  dust,  and  their  motion  being  continued, 
they  fly  off 


CHAPTER    II. 

FORCE. 

23.  MATTER  is  constantly  changing  its  form 
and  place.     The  most  solid  substance  will  in 
time  wear  away.     The  air  about  us  is  never  per- 
fectly still.     We  see  water  sometimes  as  ice,  sometimes  as 
a  liquid,  sometimes  as  a  vapor,  in  steam  or  clouds.     The 
earth  moves  sixty-eight  thousand  miles  every  hour.     An 
animal  or  vegetable  dies,  decays,  and  its  form  vanishes 
from  our  sight. 

24.  As  the  cause  of  all  the  changes  observed 

To  -what  causa 

do  we  attribute    to  take  place  in  the  material  world,  we  admit 

the  changes ob-  .  r  . 

served  in  mat-    the  existence  of  certain  forces,  or  agents,  which 

govern  and  control  all  matter, 
what  is  25.  FORCE  is  whatever  produces,  or  opposes 

motion  in  matter. 

what  is  MO.         ^6.  MOBILITY,  or  the  susceptibility  of  mo- 
bility?        tion,  is  that  property  whereby  a  body  admits 
of  change  of  place. 

what  are  the        ^7.  All  the  great  forces,  or  agents  in  nature, 
natur??cesia    th°se  which  produce,  or  are  the  cause  of  all  the 

changes  which  take  place  in  matter,  may  be 
enumerated  as  follows  :  INTERNAL,  or  MOLECULAR  FORCES, 
the  ATTRACTION  of  GRAVITATION,  HEAT,  LIGHT,  the  AT- 
TRACTIVE and  REPULSIVE  FORCES  of  MAGNETISM  and  ELEC- 
TRICITY, and,  finally,  a  force  or  power  which  only  exists 
in  living  animals  and  plants,  which  is  called,  VITAL  FORCE. 

Concerning  the  real  nature  of  these  forces,  we  are  entirely 
know  of°  the  iSnorant-  ^e  suppose,  or  say,  they  exist,  because  we  see 
nature  of  their  effects  upon  matter.  In  the  present  state  of  science,  it  is 

these  forces?       impossible  to  know  whether  they  are  merely  properties  of 
matter,  or  whether  they  are  forms  of  matter  itself,  existing  in  an 
exceedingly  minute,  subtile  condition,  without  weight,  and  diffused  through- 
out the  whole  universe.    The  general  opinion,  however,  among  scientific  men, 


22  WELLS'S   NATURAL   PHILOSOPHY. 

at  the  present  day,  is,  tliat  these  forces,  or  agents,  are  not  matter,  but  prop- 
erties, or  qualities,  of  matter. 

"We  see  a  stone  fall  to  the  ground,  and  say  that  the  cause  of  it  is  the  at- 
traction of  gravitation ; — we  observe  an  object  at  a  distance,  and  say  that  we 
see  it  through  the  action  of  light  on  the  eye  ; — we  notice  a  tree  shattered  by 
lightning,  and  say  it  is  the  effect  of  electricity ; — we  observe  an  animal  or 
plant  to  grow  and  flourish,  and  ascribe  this  to  the  action  of  the  vital  force. 
But  if  it  is  asked,  What  is  the  original  cause  of  gravitation,  light,  electricity, 
and  vital  force? — the  wisest  man  can  give  no  satisfactory  answer.  If  the 
Creator  governs  matter  through  the  agency  of  instruments,  these  forces  may 
be  called  his  agents,  or  his  instruments. 


CHAPTER    III. 

INTERNAL,    OR   MOLECULAR    FORCES. 
What  is  an  28.    AN  INTERNAL,  01  MOLECULAR  FORCE,  is 

Mo?ecuiar°r       one  ^fti  ac*s  uPon  the  particles  of  matter  only 
Force?  a^  insensible  distances.     This  variety  of  force 

differs  from  all  others  in  this  respect. 
what  is  At-  29.  The  various  changes  which  matter  un- 
dergoes,  render  it  certain  that  the  atoms,  or 
particles  of  all  bodies  are  acted  upon  by  two 
distinct  and  opposite  forces,  one  of  which  tends  to  draw 
the  atoms,  or  particles,  close  together,  while  the  other 
tends  to  separate  them  from  one  another.  The  first  of 
these  forces  we  call  ATTRACTION,  the  second  KEPULSION, 
both  acting  at  insensible  distances. 

Giv  A  blade  of  steel,  or  a  thin  piece  of  wood,  when  bent  within 

ample  of  At-      a  certain  limit,  will,  when  the  restraint  is  removed,  restore  it- 


at  an      self>  to  its  or'gilial  form-     Tm's  takes  place  through  the  agency 
insensible  dis-      of  an  internal  force,  attracting  the  particles  together,  and  tend- 
ing to  keep  them  in  their  original  place. 

whahisEias-         30.  ELASTICITY  is  that  property  of  matter 
which  disposes  it  to  resume  its  original  form 
and  shape,  after  having  been  bent  or  compressed  by  some 
external  force. 

Elasticity,  therefore,  is  not  so  much  a  distinct  property  of  matter,  as  is 
usually  stated,  as  it  is  a  phenomenon  of  attractive  and  repulsive  forces. 


23 

Do  all  bodies  ^  bodies  possess  the  property  of  elasticity,  but  in  very 
possess  clas-  different  degrees.  There  are  some  in  which  the  atoms,  after 
bending,  or  displacement,  almost  perfectly  resume  their  former 
position.  Such  bodies  are  especially  termed  elastic,  as  tempered  steel,  India- 
'  rubber,  ivory,  etc.  Other  bodies,  like  iron,  lead,  etc.,  are  elastic  in  a  limited 
degree,  not  being  able  to  bear  any  great  displacement  of  their  atoms  without 
breaking,  or  permanent  disarrangement.  Putty,  moibt  clay,  and  similar  bodies, 
possess  a  very  slight  degree  of  elasticity. 

31.  If  we  compress  a  certain  quantity  of  gas,  as  common 
^wTie^f  re"  a'ir'  an<^  ^en  a^ow  ^  *°  dilate,  by  removing  all  restraint,  it 
puision  acting  will  expand  without  limit,  and  fill  every  really  empty  space 

which  is  °?en  to  iL  This  takes  Place  through  the  agency  of 
an  internal  force  which  tends  to  drive  the  particles  from  one 
another.  There  are  many  reasons  which  lead  us  to  suppose  that  the  repuls- 
ive force  which  tends  to  keep  the  particles  of  matter  asunder  is  the  agent 
known  as  heat.  Gases  may  be  considered  as  perfectly  elastic. 

32.  According  as  the  attractive  or  repulsive 

In  what  three  °  .  _ 

forms  or  con-    iorces  prevail,  all  bodies  will  assume  one  01 

ditions  does  *,  _.    .  ,.  M. 

an  matter  ex-    three    forms    or    conditions — the    SOLID,   the 
LIQUID,  or  the  AERIFORM,*   or   GASEOUS  con- 
dition. 

what  is  a  33.  A  SOLID  body  is  $Tie  in  which  the  par- 

ticles of  matter  are  attracted  so  strongly  to- 
gether, that  the  body  maintains  its  form,  or  figure,  under 
all  ordinary  circumstances. 

what  is  a  34.  A  LIQUID  body  is  one  in  which  the  par- 

Liquid?  tides  of  matter  are  so  feebly  attracted  together, 
that  they  move  upon  each  another  with  the  greatest 
facility. 

Hence  a  liquid  can  never  be  made  to  assume  any  particular  form,  except 
that  of  the  vessel  in  which  it  is  inclosed. 

what  is  a  ^'  ^n  ABRIFORM?  or  GASEOUS  body  is  one 

in  which  the  particles  of  matter  are  not  held 
together  by  any  force  of  attraction,  but  have  a 

tendency  to  separate  and  move  off  from  one  another. 

A  gaseous  body  is  generally  invisible,  and,  like  the  air  sur- 
properties  of  a  rounding  us,  affords  to  the  sense  of  touch  no  evidence  of  its 
Gaseous  existence  when  in  a  state  of  complete  repose.  Gaseous  bodies 


Body? 


may  be  confined  in  vessels,  from  whence  they  exclude  liquids, 
Aeriform,  having  the  form,  or  resemblance,  of  air. 


24  WELLS'S   NATURAL   PHILOSOPHY. 

or  other  bodies,  thus  demonstrating  their  existence,  though  invisible,  and  also 
their  impenetrability. 

36.  Most  substances  can  be  made  to  assume 

Under  what  -1,1        /•  /•  TT          i  •        •  i 

circumstances     successively  the  lorm  of  a  solid,  a  liquid,  or  a 

will  a  bod}' as-  T  , .  ,  -,  .  „  .          , 

Bume  the  form     gas.      In   solids,    the   attractive   torce   is   the 

of  a   Solid,  a  .  •,  ,  •    i        i  ,  i      •         i  i 

Liquid,  or  a  strongest  ;  the  particles  keep  their  places,  and 
the  solid  retains  its  form.  But  if  we  heat  the 
solid  to  a  sufficient  degree,  as,  for  example,  a  piece  of  iron, 
we  gradually  destroy  the  attractive  force,  and  the  repul- 
sive force  increases  ;  the  particles  become  movable,  and  we 
say  the  body  melts,  or  becomes  a  liquid.  In  liquids,  the 
attractive  and  repulsive  forces  are  nearly  balanced,  but  if 
we  supply  an  additional  quantity  of  heat,  we  destroy  the 
attractive  force  altogether,  and  the  liquid  changes  to  a 
gas,  in  which  the  repulsive  force  prevails,  and  the  particles 
tend  to  fly  off  from  each  other.  By  the  withdrawal  of 
heat  (i.  e.,  by  the  application  of  cold),  we  can  diminish,  or 
destroy  the  repulsive  force,  and  allow  the  attractive  force 
to  again  predominate. 

Thus  steam,  when  cooled,  becomes  a 
liquid,  water;  and  this  in  turn,  by  the 
withdrawal  of  an  additional  amount  of 
heat,  becomes  a  solid,  ice. 

The  power  of  the  repulsive  force  is  strik- 
ingly illustrated  by  the  conversion  of  water 
into  steam.  In  a  cubic  inch  of  water  con- 
verted into  steam,  the  particles  will  repel 
each  other  to  such  an  extent,  that  the  space 
occupied  by  the  steam  will  be  1700  times 
greater  than  that  occupied  by  the  water. 
Fig.  2  illustrates  the  comparative  difference 
between  the  bulk  of  steam  and  the  bulk  of 
water. 

37.  The   term  FLUID  is   applied   to   those 
bodies  whose    particles    move    easily  among 

themselves.     It   is   used  to   designate   either   liquids   or 

gases. 

what  are  the        ^8.  We  distinguish  FOUR  kinds  of  molecular 

MoTec^/At-    attraction,  or  attraction  acting  upon  the  par- 

traction?          tides  of  bodies  at  insensible  distances.     These 


INTERNAL,  OR  MOLECULAR  FORCES.        25 

are,  COHESION,  ADHESION,  CAPILLARY  ATTRACTION,  and 
AFFINITY. 

39.  COHESION,  or  COHESIVE  ATTRACTION,  is 

What  is    Co-  '      . 

hesiveAttrac-     that  force  which  bmds  together  atoms  of  the 
same  kind  to  form  one  uniform  mass. 

The  force  which  holds  together  the  atoms  of  a  mass  of  iron,  wood,  or  stone, 
is  cohesion,  and  the  atoms  are  said  to  cohere  to  each  other. 

what  is  Ad-        40.  ADHESION   is   that   form    of  attraction 
which  exists  between  unlike  atoms,  or  particles 
of  matter,  when  in  contact  with  each  other. 

Dust  floating  in  the  air  sticks  to  the  wall  or  ceiling,  through  the  force  of 
adhesion.  When  we  write  on  a  wall  with  a  piece  of  chalk,  or  charcoal,  the 
particles,  worn  off  from  the  material,  stick  to  the  wall  and  leave  a  mark, 
through  the  force  of  adhesion.  Two  pieces  of  wood  may  be  fastened  together 
by  means  of  glue,  in  consequence  of  the  adhesive  attraction  between  the  par- 
ticles of  the  wood  and  the  particles  of  glue. 

41.  CAPILLARY  ATTRACTION  is  that  form  of 

What   is    Ca-  .  .  . 

piiiaryAttrac-     attraction  which  exists  between  a  liquid  and 
the  interior  of  a  solid,  which  is  tubular,  or 
porous. 

When  one  end  of  a  sponge,  or  a  lump  of  sugar  is  brought  into  contact  with 
water,  the  liquid,  by  capillary  attraction,  will  rise,  or  soak  up  above  its  level, 
into  the  interior  of  the  sponge,  or  sugar,  until  all  its  pores  are  filled.* 

what  is  Af-        42.  AFFINITY  is  that  form  of  attraction  which 
unites  atoms  of  unlike  substances  into  com- 
pounds possessing  new  and  distinct  properties. 

Oxygen,  for  example,  unites  with  iron,  and  forms  iron-rust,  a  substance 
different  from  either  oxygen  or  iron.  The  consideration  of  the  attraction  of 
Affinity  belongs  wholly  to  Chemistry. 

HOW  does  the        43.  The  force,  or  strength  of  Cohesive  At- 
B?veelftSc-e"     traction  varies  greatly  in  different  substances, 
according   as  the  nature,  form,  and  arrange- 
ment of  the  atoms  of  which  they  are  composed  vary.         •> 
44.  These  modifications  of  the  force  of  At- 

What  proper- 

ties  of  bodies     traction,  acting  at  insensible  distances  between 

depend  on  the  &  . 

variation  of      the  atoms  of  different  substances,  give  rise  to 

Attraction?  -  .  ,  ,.   , 

certain  important  properties  in  bodies,  which 
are  designated  under  the  names  of  MALLEABILITY,  Duc- 

•  Capillary  Attraction  ia  treated  of  more  fully  under  the  department  of  Hydrostatic* 
and  Hydraulics. 

2 


26  WELLS'S  NATURAL  PHILOSOPHY. 

TILITY,  PLIABILITY.  FLEXIBILITY,  TENACITY,  HARDNESS, 
and  BRITTLENESS. 

These  are  not,  as  is  often  taught,  distinct,  independent  properties  of  matter, 
like  magnitude,  porosity,  inertia,  etc.,  but  modifications  of  the  force  of  attraction. 

what  is  Mai-         45.  MALLEABILITY  is  that  property  in  virtue 
of  which  a  substance  can  be  reduced  to  the 
form  of  thin  leaves,  or  plates,  by  hammering,  or  by  means 
of  the  intense  pressure  of  rollers. 

In  malleable  bodies,  the  atoms  seem  to  cohere  equally  in  whatever  relative 
situations  they  happen  to  be,  and  therefore  readily  yield  to  force,  and  change 
their  positions  without  fracture,  almost  like  the  atoms  of  a  fluid. 

The  property  of  malleability  is  possessed  in  the  most  eminent 

anMes^of eX"      degree  by  the  metals ;  gold,  silver,  iron,  and  copper  being  the 

Malleability  ?       most  malleable.    'Gold  may  be  hammered  to  such  a  degree  of 

thinness,  as  to  require  360,000  leaves  to  equal  an  inch  in 

thickness. 

what  is  DUC-        46.  DUCTILITY  is   that   property  in   virtue 
of  which  a  substance  admits  of  being  drawn 
into  wire. 

"We  might  suppose  that  ductility  and  malleability  would  belong  to  the  same 
substances,  and  to  the  same  degree,  but  they  do  not.  Tin  and  lead  are 
highly  malleable,  and  are  capable  of  being  reduced  to  extremely  thin  leaves, 
but  they  are  not  ductile,  since  they  can  not  be  drawn  into  fine  wire.  Some 
substances  are  both  ductile  and  malleable  in  the  highest  degree.  Gold  has 
been  drawn  into  wire  so  fine,  that  an  ounce  of  it  would  extend  fifty  miles. 

what  are  47.  FLEXIBILITY  and  PLIABILITY  are  those 

fn^puabfi  properties  which  permit  considerable  motion 
ily?  of  the  particles  of  a  body  on  each  other,  with- 

out breaking. 
what  is  TC-        48.  TENACITY  is  that  property  in  virtue  of 

which  a  body  resists  separation  of  its  parts,  by 
extension  in  the  direction  of  its  length, 
what  is  49.  HARDNESS  is  a  property  in  virtue   of 

which  the  particles  of  a  body  resist  impression, 
separation,  or  the  action  of  any  force  which  tends  to  change 
their  form,  or  arrangement. 

when  is  a  50.  A  body,  whose  particles  can  be  removed, 

body  soft?  and  change(i  jn  position,  by  a  slight  degree  of 
force,  is  said  to  be  soft.  SOFTNESS  is;  therefore,  the  oppo- 
site of  hardness. 


27 

The  property  of  Hardness  is  quite  distinct  from  Density.  Gold  and  lead 
possess  great  density,  yet  they  are  among  the  softest  of  metals. 

what  is  Brit-        "51.  BRITTLENESS  is  a  property  in  virtue  of 
which  bodies  are  easily  broken  into  fragments. 
It  is  a  characteristic  of  most  hard  substances. 

In  a  brittle  body,  the  attractive  force  between  the  atoms  exists  within  such 
narrow  limits,  that  a  very  slight  change  of  position,  or  increase  of  distance 
among  them,  is  sufficient  to  overcome  it,  and  the  body  breaks. 

52.  The  modifications  of  the  force  of  cohesive  attraction  between  the  par- 
ticles of  matter,  which  give  rise  to  the  properties  of  malleability,  ductility, 
flexibility,  pliability,  hardness,  and  brittleness,  seem  to  be  intimately  con- 
nected with,  or  depend  upon  the  particular  form  of  the  atoms  of  the  sub- 
etance,  and  the  particular  manner  in  which  they  are  arranged.  * 

Every  one  knows  that  it  is  easier  to  split  wood  lengthwise  than  across  the 
fibers ;  hence,  the  force  which  binds  the  particles  of  the  wood  together  is  ex- 
erted in  a  less  degree  in  one  direction  than  in  the  other. 
Explain  how  ^v  changing  the  form  or  arrangement  of  the  atoms  of  a 
the  force  of  substance,  we  can  in  many  instances  apparently  renew  or  dc- 
pends^on  ?he  strov  tne  various  modifications  of  the  attractive  force.  The 
arrangement  following  is  a  familiar  illustration  of  this  principle : 

Steel,  when  heated  and  suddenly  cooled,  is  rendered  not 
only  very  hard,  but  very  brittle ;  but  if  heated  and  cooled  gradually,  it  be- 
comes soft  and  flexible.  We  may  suppose  that  when  the  atoms  of  steel  are 
expanded — forced  apart  from  each  other  by  the  action  of  heat,  and  then  sud- 
denly caused  to  contract — forced  in  upon  each  other — by  cooling,  that  no  op- 
portunity is  afforded  them  for  arrangement  in  a  natural  manner.  But  when 
the  steel  is  cooled  slowly,  each  atom  has  an  opportunity  to  take  the  place  best 
adapted  for  it,  without  interfering  with  its  neighbor.  According  to  one  ar- 
rangement of  the  atoms,  the  steel  is  brittle,  or  the  atoms  will  not  admit  of 
any  motion  among  themselves  without  breaking ;  but  according  to  a  different 
arrangement,  the  attractive  force  is  modified,  and  the  steel  is  soft  and  flexible. 
In  a  similar  banner,  bricks  stacked  up  irregularly,  may  be  made  to  fall 
easily,  but  if  piled  in  a  regular  manner,  they  retain  their  stability. 

It  is  a  very  singular  circumstance,  that  the  same  operation  of  heating  and  cool- 
ing suddenly,  which  hardens  steel,  should  soften  copper.  A  piece  of  steel  which 
has  been  hardened  in  this  way  is  not  condensed — made  smaller — as  we  might 
have  supposed  it  would  be,  but  is  actually  expanded,  or  made  larger.  This  proves 
that  the  arrangement  of  the  afoms,  or  particles,  has  been  changed.  Any  ono 
may  satisfy  himself  of  this  by  taking  a  piece  of  steel,  fitting  it  exactly  into  a 
guage,  or  between  two  fixed  points,  and  then  hardening  it.  It  will  then  be 
found  that  the  steel  will  not  go  into  the  guage,  or  between  the  fixed  points. 

what  is  An-         53.  The  process  of  rendering  metals,  glass, 
etc.,  soft  and  flexible  by  heating  and  gradually 
cooling,  is  called  ANNEALING,  and  is  of  great  importance 
in  the  arts. 


28  WELLS'S   NATURAL   PHILOSOPHY. 

For  example,  the  workman,  in  fashioning  and  shaping  a  steel  instrument, 
requires  it  to  be  soft  and  flexible ;  but  in  using  it  after  it  has  been  constructed, 
as  for  the  cutting  of  stone,  wood,  etc.,  it  is  necessary  that  it  should  be  hard. 
This  is  accomplished  by  making  the  steel  soft  by  annealing,  and  then  render- 
ing it  hard  by  heating  and  cooling  quickly.* 

when  will  a  54.  When  we  bend  or  compress  a  body  so 
compVesesed,°r  that  its  particles  are  separated  beyond  a  certain 
limited  distance,  the  force  of  cohesive  attrac- 
tion existing  between  them  ceases  to  act,  or  is  destroyed, 
and  the  body  falls  apart,  or  breaks. 

55.  When  the  Attraction  of  Cohesion  between 

Can    we     re- 
store the  at-   »the  particles  of  a  substance  is  once  destroyed, 

traction  of  co-  .  r  *L 

dest?!!  el?6"     *  1S  generalty  impossible  to  restore  it.     Hav- 
ing once  reduced  a  mass  of  wood  or  stone  to 
powder,  we  can  not  make  the  minute  particles  cohere 
again  by  pushing  them  into  their  former  position. 

In  some  instances,  however,  this  can  be  accomplished  by  resorting  to  va- 
rious expedients.  The  particles  of  the  metals  may  be  made  to  again  cohere 
by  melting.  Two  pieces  of  perfectly  smooth  plate-glass,  or  marble,  laid  upon 
each  other,  unite  together  with  such  force,  that  it  is  impossible  to  separate 
them  without  breakage.  In  the  manufacture  of  looking-glass  plates,  this  at- 
traction between  two  smooth  surfaces  is  particularly  guarded  against 

*  There  are  many  practical  illustrations  in  the  arts,  of  the  principle,  that  the  modifica- 
tions of  the  attractive  force  which  unites  the  atoms  of  solid  bodies  together,  are  dependent 
in  a  great  degree  upon  the  forms,  or  arrangement  of  the  atoms  themselves.  If  we  submit 
apiece  of  metal  to  repeated  hammering,  or  jarring,  the  atoms,  or  particles  of  which  it  is 
composed,  seem  to  take  on  a  new  arrangement,  and  the  metal  gradually  loses  all  its  te- 
nacity, flexibility,  malleability,  and  ductility,  and  becomes  brittle.  The  coppersmith  who 
forms  vessels  of  brass  and  copper  by  the  hammer  alone,  can  work  on  them  only  for  a  short 
time  before  they  require  annealing ;  otherwise  they  would  crack  and  fly  into  pieces. 

For  this  reason,  also,  a  cannon  can  only  be  fired  a  certain  number  of  times  before  it 
will  burst,  and  a  cannon  which  has  been  long  in  use,  although  apparently  sound,  is  always 
condemned  and  broken  up. 

A  more  important  illustration,  and  one  that  more  closely  affects  our  interests,  is  the 
liability  of  railroad  car-axles  and  wheels  to  break  from  the  same  cause.  A  car-axle,  after 
a  long  lapse  of  time  and  use,  is  almost  certain  to  break. 

That  these  phenomena  are  due  to  changes  in  the  manner  of  the  arrangement  and  the 
form  of  the  particles,  or  atoms,  of  matter,  was  conclusively  proved  by  an  experiment  made* 
a  few  years  since  in  France  :— An  accident  having  occurred  upon  a  railroad,  by  the  break- 
ing of  an  axle,  by  which  many  lives  were  lost,  the  attention  of  scientific  men  was  called 
to  the  fact,  that  the  iron  composing  the  axle,  when  first  used,  was  strong,  and  capable  of 
standing  a  test,  but  after  use  in  locomotion  for  a  certain  period,  could  be  broken  by  a 
force  far  inferior  to  that  by  which  it  had  formerly  been  tested.  Many  suppositions  were 
made  to  account  for  this  phenomenon,  when  finally  a  person  took  a  series  of  rods  about  the 
size  of  pipe-stems,  all  strong  and  tough,  and,  with  great  patience,  allowed  them  to  fall 
for  hours  and  hours  upon  an  anvil,  thus  producing  rapid  strokes  and  vibrations.  After 
subjecting  them  for  a  long  time  to  this  treatment,  he  found  that  the  rods  could  be  snap- 
ped and  broken  into  fragments  almost  as  easily  as  rotten  wood. 


OR   MOLECULAR   FORCES.  29 

what  is  56.  Iron  may  be  made  to  cohere  to  iron  by 

heating  the  metal  to  a  high  degree,  and  ham- 
mering the  two  pieces  together.  The  particles  are  thus 
driven  into  such  intimate  contact,  that  they  cohere  and 
form  one  uniform  mass.  This  property  is  called  WELD- 
ING, and  only  belongs  to  two  metals,  iron  and  platinum. 

PRACTICAL  QUESTIONS  ON  THE  INTERNAL,  OR  MOLECULAR 

FORCES. 

1.  In  what  respect  does  a  gas  DIFFER  from  a  liquid  ? 

A  liquid,  like  water,  milk,  syrup,  etc.,  can  be  made  to  flow  regularly  down 
a  slope,  or  an  inclined  plane,  but  a  gas  can  not.  \  rw  On  ; 

2.  Why  is  a  har  of  IBON  stronger  than  a  bar  of  WOOD  of  the  same  size  ? 

Because  the  cohesion  existing  between  the  particles  of  iron  is  greater  than 
that  existing  between  the  particles  of  wood. 

3.  Why  are  the  particles  of  a  LIQUID  more  easily  separated  than  those  of  a  SOLID? 
Because  the  cohesive  attraction  which  binds  together  the  particles  of  a  liquid 

is  much  less  strong  than  that  which  binds  together  the  particles  of  a  solid. 

4.  Why  will  a  small  needle,  carefully  laid  upon  the  surface  of  water,  FLOAT  ? 
Because  its  weight  is  not  sufficient  to  overcome  the  cohesion  of  the  particles 

of  water  constituting  the  surface  ;  consequently,  it  can  not  pass  through  them 
and  sink. 

5.  If  you  drop  water  and  laudanum  from  the  same  vessel,  why  will  SIXTY  drops  of  the 
water  fill  the  same  measure  as  OXK  HUNDEED  drops  of  laudanum? 

The  cohesion  between  the  particles  of  the  two  liquids  is  different,  being 
greatest  in  the  water.  Consequently,  the  number  of  particles  which  will  ad- 
here together  to  constitute  a  drop  of  water,  is  greater  than  in  the  drop  of 
laudanum. 

6.  Why  is  the  prescription  of  medicine  by  DBOPS  an  unsafe  method  ? 

Because,  not  only  do  drops  of  fluid  from  the  same  vessel,  and  6ften  of  the 
same  fluid  from  different  vessels,  differ  in  size,  but  also  drops  of  the  same  fluid, 
to  the  extent  of  a  third,  from  different  parts  of  the  lip  of  the  same  vessel. 

7.  Why  are  cements  and  mortars  used  to  fasten  bricks  and  stone  together? 
Because  the  adhesive  attraction  between  the  particles-  of  brick  and  stone 

and  the  particles  of  mortar,  is  so  strong,  that  they  unite  to  form  one  solid 


8.  How  may  the  efficacy  of  a  locomotive  engine  be  said  to  depend  upon  the  force  of 
adhesion  ? 

If  there  were  no  adhesion,  or  even  insufficient  adhesion,  between  the  tire 
of  the  driving-wheel  of  the  locomotive,  and  the  rails  upon  which  it  presses, 
the  wheel  would  turn  without  advancing. 

This  actually  happens  when  the  rails  are  greasy,  or  covered  with  frost  and 


30  WELLS'S   NATURAL   PHILOSOPHY. 

ice.     The  contact  is  thus  interrupted,  and  the  adhesion  between  the  rail  and 
wheel  is  impaired. 

9.  When  a  liquid  adheres  to  a  solid,  -what  term  do  we  apply  to  designate  the  act  of 
adhesion  ? 

Wetting.  It  is  necessary  that  a  liquid  should  adhere  to  the  surface  of  a  solid 
before  it  can  be  wet.  Water  falling  upon  an  oiled  surface  does  not  wet  it, 
because  there  is  no  adhesion  between  the  particles  of  the  oil  and  the  particles 
of  the  water. 

10.  Why  are  drops  of  rain,  of  tears,  and  of  dew  upon  the  leaves  of  plants,  generally 
spherical,  or  globular  ?  I 

The  force  of  cohesion  always  tends  to  cause  the  particles  of  a  liquid,  when 
unsupported,  or  supported  on  a  surface  having  little  attraction  for  it,  to  as- 
sume the  form  of  a  sphere — a  globe,  or  sphere,  being  the  figure  which  will 
contain  the  greatest  amount  of  matter  within  a  given  surface. 

This  property  of  fluids  is  taken  advantage  of  in  the  arts,  in  the  manufacture 
of  shot.  The  melted  lead  is  made  to  fall  in  a  shower,  from  a  great  elevation. 
In  its  descent  the  drops  become  globular,  and  before  they  reach  the  end  of 
their  fall  become  hardened  by  cooling,  and  retain  their  form. 


CHAPTER    IV. 

ATTRACTION  OF  GRAVITATION. 

57.  THE  ATTRACTION  of   GRAVITATION  is 
Avi    that  form  of  attraction,  by  which  all  bodies  at 

Gravitation?      gensible  distances,  tend  to  approach  each  other. 

Electricity  and  Magnetism  attract  bodies  at  sensible  dis- 

GravitatiOTi         tances  also,  but  their  influence  upon  different  classes  of  bodies 

differ  from         varies,  and  is  limited  by  distance.     Molecular,  or  Internal  At- 

of  attraction?       traction,  acts  only  at  insensible  distances.     The  Attraction  of 

Gravitation  acts  at  all  distances,  and  upon  all  bodies. 

58.  Every  portion  of  matter  in  the  universe 

What   is   the  J    *  .  .          „ 

great  law  of     attracts  every  other  portion,  with  a  force  pro- 

the  attraction  ..     ,.  J  .         f      , 

of    oravita-    portioned  directly  to  its  mass,  or  quantity,  and 
inversely  as  the  square  of  the  distance.     This 
is  the  great  general  law  of  the  Attraction  of  Gravitation. 

By  the  Attraction  of  Gravitation  being  directly  proportional  to  the  mass  of 
a  body,  we  mean,  that  if  of  two  bodies,  the  mass  of  one  be  twice  as  large  as 
that  of  the  other,  its  force  of  attraction  will  be  twice  as  great :  if  it  is  only 
half  as  large,  its  attraction  will  be  only  half  as  great. 

By  the  Attraction  of  Gravitation  being  inversely  proportioned  to  the  square 


ATTRACTION   OF   GRAVITATION.  31 

of  the  distance,  we  mean,  that  if  one  body,  or  substance,  attracts  another  body 
with  a  certain  force  at  the  distance  of  a  mile,  it  will  attract  with  four  times 
that  force  at  half  a  mile,  nine  times  the  force  at  one  third  of  a  mile,  and  so 
on,  in  like  proportion.  On  the  contrary,  it  will  attract  with  but  one  fourth  of 
the  force  at  two  miles,  one  ninth  of  the  force  at  three  miles,  one  sixteenth  of 
the  force  at  four  miles,  and  so  on,  as  the  distance  increases. 

JIJG  3  This  law  may  be  further 

illustrated  by  reference  to 
Pig.  3.  Let  C  be  the  center 
of  attraction,  and  let  the  four 
dotted  lines  diverging  from  C 
represent  lines  of  attraction. 
At  a  certain  distance  from  G 
they  will  comprehend  the 
small  square  A ;  at  twice  that 

distance  they  will  include  the  large  square  B,  four  times  the  size  of  A ;  and 
since  there  is  only  a  certain  definite  amount  of  attraction  included  within 
these  lines,  it  is  clear  that  as  B  is  four  times  as  great  as  A,  the  attraction  ex- 
erted upon  a  portion  of  B  equal  to  A,  will  be  only  one  fourth  that  which  it 
would  experience  when  in  the  position  marked  1,  just  half  as  far  from  C. 

As  gravitative  attraction  is  the  common  property  of  all 
IKdfea  up-  bodies»  it  ma7  be  asked,  why  all  bodies  not  fastened  to  the 
on  the  earth' a  earth's  surface  do  not  come  in  contact?  They  would  do  so, 
incfontactTe  were  [i  not  for  the  overpowering  influence  of  the  earth's  at- 
traction, which  in  a  great  measure  neutralizes,  or  overcomes, 
the  mutual  attraction  of  smaller  bodies  on  its  surface. 

Does  a  feather  ^e  throw  UP  a  feather  into  the  air,  and  it  falls  through  the 
attract  the  influence  of  the  earth's  attraction ;  but  as  all  bodies  attract 
each  other,  the  feather  must  also  attract,  or  draw  up,  the 
earth,  in  some  degree,  toward  itself.  This  it  really  does,  with  a  force  pro- 
portioned to  its  mass ;  but  as  the  mass  of  the  earth  is  infinitely  greater  than 
the  mass  of  the  feather,  the  influence  of  the  feather  is  infinitely  small,  and  we 
are  unable  to  perceive  it. 

What  are  il  In  S°m6  instances>  where  bodies  are  free  to  move,  the  mu- 

lustrations  of  tual  attraction  of  all  matter  exhibits  itself.  If  we  place  upon 
Mutual  At-  water,  in  a  smooth  pond,  two  floating  bodies  at  certain  dis- 
tances from  each  other,  they  will  eventually  approach,  the  con- 
ditions affecting  the  experiment  being  alike  for  each.  Two  leaden  balls  sus- 
pended by  a  string  near  each  other,  are  found,  by  delicate  tests,  to  attract 
each  other,  and  therefore  not  to  hang  quite  perpendicular.  A  leaden  weight 
suspended  near  the  side  of  a  mountain,  inclines  toward  it  to  an  extent  pro- 
portionate to  the  magnitude  of  the  mountain. 

What  is  the          TIie  earth  afctracts  the  moon,  and  this  in  turn  attracts  the 

cause     of  earth.    The  solid  particles  of  matter  upon  the  earth's  surface, 

not  being  free  to  move,  do  not  sensibly  show  the  influence  of 

the  moon's  attraction ;  but  the  particles  of  water  composing  the  ocean,  being 


32  WELLS'S   NATURAL   PHILOSOPHY. 

free  to  move,  furnish  us  evidence  of  this  attraction,  in  the  phenomena  of  the 
tides.  "When,  by  the  revolution  of  the  earth,  a  certain  portion  of  its  surface 
is  brought  within  the  direct  influence  of  the  moon's  attraction,  the  surface  of 
the  ocean  is  attracted,  or  drawn  up,  to  form  a  wave.  This  wave,  or  elevation 
of  the  surface  of  the  water,  occurring  uniformly,  is  called  a  tide  ;  when  the 
moon  is  the  nearest  to  the  earth*  its  attraction  is  the  greatest,  and  at  these 
periods  we  have  high  tides,  or  "  high  water." 

what  is  Ter  ^'  "^  Bodies  uPon  the  earth  are  attracted 
restriai  Gray-  toward  its  center.  This  we  call  Terrestrial 

ity? 

Gravitation. 

what  is  the  The  attraction  of  the  earth  is  not  the  same 
eaTth'  sittr&c-  at  all  distances  from  the  center,  being  greatest 

at  the  surface,  and  decreasing  upward  as  the 
square  of  the  distance  from  the  center  increases,  and  down- 
ward simply  as  the  distance  from  the  center  decreases. 

SECTION    I. 

WEIGHT. 

60.  When  a  body  falls  to  the  earth,  it  de- 

How  is  a  body  .  J 

at  rest  upon     scends  because  it  is  attracted  toward  the  center 

the  surface  of  ,  TTTI  •  11  f  f 

the  earth  at-  of  the  earth.  When  it  reaches  the  surface  of 
the  earth,  and  rests  upon  it,  its  tendency  to 
continue  to  descend  toward  the  center  is  not  destroyed, 
and  it  presses  downwards  with  a  force  proportioned  to  the 
degree  by  which  it  is  attracted  in  this  direction.  This 
pressure  we  call  Weight. 

what  is  61.  Weight  is,  therefore,   the  measure   of 

force  with  which  a  body  is  attracted  by  the 
earth.  In  ordinary  language,  it  is  the  quantity  of  matter 
contained  in  a  body,  as  ascertained  by  the  balance. 

Weight  being,  then,  the  measure  of  the  earth's  attraction,  it 
WeT  hf  va    ?      follows  that  as  tbe  attraction  of  the  earth  varies,  w?ight  must 
also  vary,  or  a  body  will  not  have  the  same  weight  at  all 
places. 

The  weight  of  a  body  will  be  greatest  at  the  surface  of  the 
earth»  and  S™**^  at  those  Points  uPon  the  surface  which  are 


by     wgh 

the  most,  and      nearest  the  center. 


,  . 

the  As  the  earth  is  not  a  perfect  sphere,  but  fattened  at  the 

poles,  the  poles  are  nearer  the  center  than  the  equator.    A 


WEIGHT.  33 

body,  therefore,  will  be  attracted  most  strongly,  that  is,  will  weigh  the  most, 
at  the  poles,  or  at  that  portion  of  the  earth's  surface  which  is  nearest  the 
center,'  and  weigh  the  least  at  the  equator,  or  at  that  portion  of  the  earth's 
surface  which  is  most  remote  from  the  center. 

A  ball  of  iron  weighing  one  thousand  pounds  in  the  latitude  of  the  city  of 
New  York,  at  the  level  of  the  sea,  will  gain  three  pounds  in  weight,  if  re- 
moved to  the  north  pole,  and  lose  about  four  pounds  if  conveyed  to  the 
equator. 

HOW  does  62.  If  a  body  be  lifted  above  the  surface  of 

Ts^aslend  the  earth,  its  weight  will  decrease  in  accord- 
eI°rth'stheBur-  ance  with  the  law,  that  the  attraction  of 
gravitation  decreases  upward  from  the  surface, 
as  the  square  of  the  distance  from  the  center  of  the  earth 
increases. 

The  weight  of  a  body,  therefore,  will  be  four  times  greater  at  the  earth's 
surface,  than  at  double  the  distance  of  the  surface  from  the  center  ;  or  a  body- 
weighing  one  pound  at  the  earth's  surface,  will  have  only  one  fourth  of  that 
weight,  if  removed  as  far  from  the  surface  of  the  earth,  as  the  surface  is  from 
the  center. 

HOW  does  63.  As  the  at  traction  of  gravitation  decreases 

weight    vary    downward  from  the  surface  to  the  center  of  the 

as  we  descend  .  . 

face?  the  8ur"     eartn?  simply  as  the  distance  decreases,  weight 
will  decrease  in  like  manner. 

A  body  weighing  a  pound  at  the  surface  of  the  earth,  will  weigh  only  half 
a  pound  at  one  half  the  distance  from  the  surface  to  the  center. 

where  will  a  ^4.  At  the  center  of  the  earth  a  body  will 
u°  necessarity  lose  all  weight,  since,  being  sur- 
rounded on  all  sides  by  an  equal  quantity  of 
matter,  it  will  be  attracted  .equally  in  all  directions,  and, 
therefore,  can  not  exert  a  pressure  greater  in  one  direction 
than  in  another. 

What  are  -^-s  ^e  attractive  force  which  the  earth  exerts  upon  a  body 


d*n<«  is  Pr°P°rtioned  to  its  mass>  cr  to  tne  quantity  of  matter  con- 
tained in  it,  and  as  weight  is  merely  the  measure  of  such  at- 
traction, it  follows  that  a  body  of  a  large  mass  will  be  attracted  strongly,  and 
possess  great  weight,  while,  on  the  contrary,  a  body  made  up  of  a  small 
quantity  of  matter,  will  be  attracted  in  a  less  degree,  and  possess  less  weight. 
We  recognize  this  difference  of  attraction  by  calling  the  one  body  heavy  and 
the  other  light. 

1  If,  as  is  represented  in  Fig.  4,  we  place  a  mass  of  lead,  a,  at  one  extremity 
of  a  well-balanced  beam,  and  a  feather,  6,  at  the  other,  we  shall  find  that  the 

2* 


34  WELLS'S  NATURAL  PHILOSOPHY. 

lead  is  drawn  to  the  earth  with  a  force  exactly  equal  to  the  superiority  of  its 
j'IGt  4<  mass  over  that  of  the  feather.    If, 

however,  we  tie  on  a  sufficient 
number  of  feathers  to  make  up  a 
quantity  of  matter  equal  to  that 
of  the  lead,  the  equilibrium  is  re- 
stored— the  two  quantities  are 
attracted  with  equal  force,  and 
the  beam  is  supported  in  a  hori- 
zontal position. 

65.  In  all  the  opera- 
tions of  trade  and  com- 
merce, we    sell,   or  ex- 
O*  ;  change  a  given  quantity 

\  j  of  one  article  or  substance 
for  a  certain  quantity  of 
some  other  article  or  substance — so  much  flour  for  so  much 
sugar,  or  so  much  sugar  and  flour  for  so  much  gold, 
what  is  a  sys.  Hence  the  necessity,  which  has  existed  from 
and  Mrawures?  ^G  earliest  ages,  of  having  some  fixed  rules  or 
standards,  according  to  which  different  quanti- 
ties of  different  substances  may  be  compared.  A  set,  or 
series,  of  such  rules  or  standards  of  comparison,  is  called  a 
System  of  Weights  and  Measures. 

What  are  the  Various  nations  adopt  different  standards,  but  hi  the  civil- 
two  great  Sys-  ized  and  commercial  world,  but  two  great  Systems  of  Weights 
Weights  and  an(*  Measures  are  generally  recognized.  These  are  known  as 
Measures?  the  English,  and  the  French  Systems. 

In  the  English  System,  which  is  the  one  used  in  the  United 

States,  there  are  two  systems  of  weights — Troy  and  Avoirdu- 
the  English  pois  Weight.  Troy  Weight  is  principally  used  for  weighing 

gold  and  silver;  Avoirdupois  for  weighing  merchandize,  other 
than  the  precious  metals.  It  derives  its  name  from  the  French  avoirs  (averia), 
goods  or  chattels,  and  poids,  weight.  The  smallest  weight  made  use  of  in 
the  English  System  is  a  grain.  By  a  law  of  England  enacted  in  1286,  it 
was  ordered  that  32  grains  of  wheat,  well  dried,  should  weigh  a  pennyweight. 
Hence  the  name  grain  applied  to  this  measure  of  weight.  It  was  afterward 
ordered  that  a  pennyweight  should  be  divided  into  only  24  grains.  Grain 
weights  for  practical  purposes,  are  made  by  weighing  a  thin  plate  of  metal  of 
uniform  thickness,  and  cutting  out,  by  measurement,  such  a  proportion  of 
the  whole  as  should  give  one  grain.  In  this  way,  weights  may  be  obtained 
for  chemical  purposes,  which  weigh  only  the  1,000th  part  of  a  gram. 


WEIGHT. 


35 


HO*  do  we  ob-  *>6.  In  constructing  a  System  of  Weights 
aard  of  wet-Ms  an^  Measures,  it  is  necessary,  in  the  first  place, 
and  Measures?  fo  gx  Up0n  some  dimension  which  shall  forever 
serve  as  a  standard  from  which  all  other  weights  and 
measures  may  be  derived,  and  by  which  they  may  be  com- 
pared and  verified.  If  an  artificial  standard  were  taken, 
it  is  evident  that  it  might  be  falsified,  or  even  entirely  lost 
or  destroyed,  thus  creating  great  confusion.  It  is,  there- 
fore, necessary  to  fix  upon  some  unchanging  and  invariable 
space  or  size  in  nature,  which  will  always  serve  as  a  stand- 
ard, and  which  the  art  of  man  can  not  affect.  In  the 
English  System  of  Weights  and  Measures,  such  an  un- 
varying dimension,  or  standard,  is  found  in  the  length  of 
a  pendulum. 

Describe  the  ^7.  A  pendulum  is  a  heavy  body,  suspended 
Pendulum.  from  a  fixed  point  by  a  wire  or  cord,  in  such  a 
manner  that  it  may  swing  freely  backward  and  forward. 
The  alternate  movements  of  a  pendulum  in  opposite  di- 
rections are  called  its  VIBRATIONS,  or  OSCILLATIONS,  and 
the  part  of  a  circle  over  which  it  moves  is  called  its  ARC. 

In  Fig.  5,  A  B  represents  a  pendulum ;  D 

C,  the  arc  in  which  it  vibrates. 

Now,  it  has  been  found  that 
a  pendulum,  of  any  weight, 
which  in  the  latitude  of  Lon- 
don will  vibrate,  or  swing  over 
the  same  arc,  or  from  the 

highest  point  on  one  side,  to  the  highest  point 

on  the  other  side,  in  one  second  of  time,  will 

always,  under  the  same  circumstances,  have 

the  same  length.  The  length  of  this  pendulum 

(the  part  A  B,  Fig.  5)  is  divided  into  391,393 

equal  parts.     Of  these  parts,  1 0, 0  0  0  are  called 

an  inch,  twelve  of  which  make  one  foot, 

thirty-six  of  them  one  yard.     Thus  we  ob- 
tain standards  of  linear  measure. 

_  To  obtain  a  Standard  of  Weight,  a  cubic  inch  (accurately  ob- 

tain  a  standard     iained  from  the  pendulum}  of  distilled  water,  of  the  temperature 

of  Weight?          of  62o   Fahrenheit's  thermometer,    is  taken    and  weighed. 

This  weight  is  divided  into  252,458  equal  parts;  and  of  these,  1,000  will  be 

a  grain.    The  grain  multiplied,  gives  ounces,  pounds,  etc. 


HOTV  does  the 
Pendulum  fur- 
nish a.  Stand- 
ard of  Meas- 
ures of  Length? 


36  WELLS'S  NATURAL  PHILOSOPHY. 

Ho    .  To  obtain  standards  of  Liquid  Measure,  ten  pounds,  or  7,000 

tain  standards     grains  of  distilled  water,  at  the  same  temperature,  are  made 
Measures'?  *°  constitute  a  gallon.     The  gallon,  by  division,  gives  quarts, 

pints,  and  gills. 

68.  The  French  System  of  "Weights  and  Measures  is 
constructed  on  a  different  plan,  and  originated  in  the  fol- 
lowing manner  : 


E    lain  th  In  1^88'  *^e  Drench  Government,  feeling  the  necessity  of 

construction  of  having  some  standard  by  which  all  weights  and  measures 
the  French  might  be  compared  and  made  uniform,  ordered  a  scientific  in- 
Weights  and  quiry  to  be  made  ;  the  result  of  which  was  the  establishment 
Measures.  of  the  presen|;  system  of  -French  Weights  and  Measures,  which, 

from  its  perfect  accuracy  and  simplicity  is  superior  to  all  other  systems.  It  is 
sometimes  called  the  Decimal  System,  all  its  divisions  being  made  by  ten. 

The  French  standard  is  based  on  an  invariable  dimension  of  the  globe,  viz.,  a 
fourth  part  of  the  earth's  meridian,  or  the  fourth  part  of  the  largest  circle  pass- 
ing through  the  poles  of  the  earth. 

p  In  Fig.  6,  the  circle  N  E  S  ~W  repre- 

sents a  meridian  of  the  earth  ;  and  a  fourth 
part  of  this  circle,  or  the  distance  N  E,  con- 
stitutes the  dimension  on  which  the  French 
System  is  founded.  This  distance,  which 
was  accurately  measured,  is  divided  into 
ten  million  equal  parts  ;  and  a  single  ten 
millionth  part  adopted  as  a  measure  of 
length,  and  called  a  metre.  The  length  of 
the  metre  is  about  39  English  inches.  By 
multiplying  or  dividing  this  quantity  by  ten, 
the  other  varieties  of  weights  and  measures 
are  obtained. 

69.  In  the  United  States,  Standards  of  Weights  and 
Measures,  prepared  according  to  the  English  System  by 
order  of  the  Government,  are  to  be  found  at  Washington, 
and  at  the  capital  of  every  State. 


PRACTICAL  PROBLEMS  ON  THE  ATTRACTION  OF  GRAVI- 
TATION. 

1.  Suppose  two  bodies,  one  weighing  30  and  the  other  90  pounds,  situated  ten  miles 
apart,  were  free  to  move  toward  each  other,  under  the  influence  of  mutual  attraction : 
what  space  would  each  pass  over  before  they  came  in  contact  ? 

The  mutual  attraction  of  any  two  bodies  for  each  other  is  proportional  to  the  quantity 
of  matter  they  contain. 

2.  A  tooTly  upon  the  surface  of  the  earth  weighs  one  pound,  or  sixteen  ounces:  if  by 


SPECIFIC   GRAVITY,   OB  WEIGHT.  37 

any  means  ire  could  carry  it  4,000  miles  above  the  earth's  surface,  what  would  be  its 
weight? 

Solution:  The  force  of  gravity  decreases  upward,  as  the  square  of  the  distance  from 
the  center  increases :  weight,  therefore,  will  decrease  in  like  proportion.  The  distance  of 
the  body  upon  the  surface  of  the  earth,  from  the  center,  is  4,000  miles.  Its  distance  from 
the  center,  at  a  point  4,000  miles  above  the  surface,  is  8,000.  The  square  of  4,000  is 
16,000,000 ;  the  square  of  8,000  is  64,000,000.  The  weight,  therefore,  will  be  diminished 
in  the  proportion  that  sixty-four  bears  to  sixteen ;  that  is,  it  will  be  diminished  ^ths,  or 
weigh  :|th  of  a  pound,  or  4  ounces. 

3.  What  will  be  the  weight  of  the  same  body  removed  8,000  miles  from  the  earth's 
surface  ? 

4.  A  body  on  the  surface  of  the  earth  weighs  ten  tons:  what  would  be  its  weight  if 
elevated  2,000  miles  above  the  surface  ? 

5.  How  far  above  the  surface  of  the  earth  mast  a  pound  weight  be  carried,  to  make  it 
weigh  one  ounce  avoirdupois  ? 

6.  What  would  a  body  weighing  800  pounds  upon  the  earth's  surface,  weigh  1,000 
miles  below  the  surface? 

The  force  of  gravity  decreases  as  we  descend  from  the  surface  into  the  earth,  simply 
as  the  distance  downward  increases,— weight  being  the  measure  of  gravity,  it  therefore 
decreases  in  the  same  proportion.  The  distance  from  the  surface  of  the  earth  to  the 
center  maybe  assumed  to  be  4,000  miles:  1,000  miles  is  one  fourth  of  4,000.  The  dis- 
tance being  decreased  one  fourth,  the  weight  is  diminished  in  like  proportion,  and  the 
body  will  lose  200  pounds,  or  its  total  weight  would  be  600  pounds. 

7  Suppose  a  body  weighing  800  pounds  upon  the  surface  of  the  earth  were  sunk  3,000 
miles  below  the  surface :  what  would  be  its  loss  in  weight  ? 

8  If  a  mass  of  iron  ore  weighs  ten  tons  upon  the  earth's  surface,  what  would  it  weigh 
at  the  bottom  of  a  mine  a  mile  below  the  surface  ? 

9.  What  will  be  the  weight  of  the  same  mass  at  the  bottom  of  a  mine  one  half  a  mile 
below  the  earth' s  surface  ? 


SECTION    II. 

SPECIFIC  GRAVITY,   OB  WEIGHT. 


In    what    two  '  water'  and  ^oats  UPon 

senses  may  the     silver.     In  the  first  instance,  we  say  the  iron  sinks  because  it 


*s  neav*er  ^an  water  ;  and  in  the  second,  it  floats,  because  it 
is  lighter  than  quicksilver.  Iron,  therefore,  is  a  heavy  body 
compared  with  water,  and  a  light  body  compared  with  mercury.  But  in  or- 
dinary language,  we  always  consider  iron  as  a  heavy  body.  The  term 
weight  may,  therefore,  be  used  in  two  very  different  senses,  and  a  body  may 
be  at  once  very  light  or  very  heavy  according  to  the  sense  in  which  the  terms 
are  used.  A  mass  of  cork  which  weighs  a  ton  is  very  heavy,  because  its  ab- 
solute weight  as  indicated  by  the  balance,  viz..  2,000  pounds,  is  considerable. 
It  is,  however,  in  another  sense,  a  light  body,  because  if  compared,  bulk  for 
bulk,  with  most  other  solid  substances,  its  weight  is  very  small.  Hence  we 
make  a  distinction  between  the  absolute,  or  real  weight  of  a  body,  and  its 
specific,  or  comparative  weight. 


38  WELLS'S  NATURAL   PHILOSOPHY. 

what  is  Ab-  71.  The  ABSOLUTE  WEIGHT  of  a  body  is 
solute  weight?  tkat  of  its  entire  masSj  without  any  reference 

to  its  bulk,  or  volume. 

what  is  sPc.  72.  The  SPECIFIC  WEIGHT,  or  the  SPECIFIC 
erne  weight?  GRAVITY  of  a  body,  is  the  weight  of  a  given 
bulk,  or  volume  of  the  substance,  compared  with  the  weight 
of  the  same  bulk,  or  volume,  of  some  other  substance. 

The  term  "  Specific"  Weight,  or  Gravity,  is  used,  because 
th^tem  "CSpe!  bodies  of  different  species  of  matter  have  different  weights 
cific,"  as  ap-  under  equal  bulks,  or  volumes.  Thus,  a  cubic  inch  of  cork, 
Weight1?0  has  a  different  weight  from  a  cubic  inch  of  oak,  or  of  gold,  and 

a  cubic  inch  of  water  contains  a  less  weight  than  a  cubic  inch 
of  mercury.  Hence  we  say  that  the  specific  gravity,  or  specific  weight,  of 
cork  is  less  than  that  of  oak  or  gold,  and  the  specific  gravity  of  mercury  is 
greater  than  that  of  water. 

73.  Specific  Gravity,  or  Weight,  being  merely  the  compara- 
Staiidard*  for  tive  Sravitr'  or  weigut>  it  is  convenient  that  some  standard 
estimating  the  should  be  selected,  to  which  all  other  substances  may  be  re- 
ft^onwdies  ?"  ferred  for  comparison.  Distilled  water  has  accordingly  been 

taken,  by  common  consent,  as  the  standard  for  comparing  the 
weights  of  all  bodies  in  the  solid,  or  liquid  form.  The  reason  for  using  dis- 
tilled water  is,  that  we  may  be  certain  of  its  purity. 

Water,  therefore,  being  fixed  upon  as  the  standard,  we  determine  the  spe- 
cific gravity  of  a  body,  or  we  ascertain  how  much  heavier  or  lighter  a  sub- 
stance is  than  water,  by  the  following  rule  :  — 

HO*  do  we        74.  Divide  the  weight  of  a  given  bulk  of  the 
dfic  *  c?n5S£    substance,  by  the  weight  of  an  equal  bulk  of 
water. 


Explain  the  Suppose  we  take  five  vessels,  each  of  which  would  contain 

application  of  exactly  one  hundred  grains  of  water,  and  fill  them  respectively 

is  ru  e.  -with  spirits,  ice,  water,  iron,  and  quicksilver.  The  following 

differences  in  weight  will  be  found  :  —  The  vessel  filled  with  spirits  would 
weigh  80  grains;  with  ice,  90  grains;  with  water,  100  grains  ;  with  iron,  750 
grains  ;  with  quicksilver,  1,350  grains. 

Water  having  been  selected  as  the  standard  for  comparing  these  different 
weights,  the  question  to  be  settled  is  simply  this:  How  much  lighter  than 
water  are  spirits  and  ice,  and  how  much  heavier  than  water  are  iron  and 
quicksilver;  or,  in  other  words,  how  many  times  is  100  contained  in  80,  90, 
750,  and  1,350?  The  weights  of  the  different  substances  filling  the  vessel 
are,  therefore,  to  be  divided  by  100,  the  weight  of  the  water  ;  and  there  is 
found  for  spirits  the  weight  0'80,  one  fifth  lighter  than  water  ;  for  the  ice,  0-90, 
one  tenth  lighter  than  water;  for  the  iron,  7-50,  or  seven  and  a  half  times 
heavier  than  water;  for  the  quicksilver,  13'50,  or  thirteen  and  a  half  times 


SPECIFIC   GRAVITY,  OK  WEIGHT. 


39 


How  do  we  ob- 
tain   the   Spe- 
cific    Gravity 
of     Liquid 
bodies? 


in  water,  what 
occurs  ? 


heavier  than  water.     These  numbers,  therefore,  are  the  specific  gravities  of  the 

spirits,  ice,  iron,  and  quicksilver. 

For  obtaining  the  specific  gravity  of  Liquids  the  method 
above  described  is  substantially  the  one  usually  adopted  in  the 
arts.  A  bottle  capable  of  holding  exactly  1,000  grains  of 
distilled  water,  at  a  temperature  of  60°  Fahrenheit,  is  ob- 
tained, filled  with  water,  and  balanced  upon  the  scales.  The 

water  is  then  removed,  and  its  place  supplied  with  the  fluid  whose  specific 

gravity  we  wish  to  determine,  and  the  bottle  and  contents  again  weighed. 

The  weight  of  the  fluid,  divided  by  the  weight  of  the  water,  gives  the  specific 

gravity  required.     Thus  a  bottle  holding  1,000  grains  of  distilled  watpr,  will 

hold  1,845  grains  of  sulphuric  acid;  1,845-4-1,000  =  1.845,  or,  the  sulphuric 

acid  is  1.845  times  heavier  than  an  equal  bulk  of  water. 

When  we  im  For  ol;)tainiDg  tne  specific  gravity  of  solid  bodies,  a  different 

merse  a  body  method  is  adopted.  When  we  immerse  a  body  in  water, 
it  displaces  a  quantity  of  water  equal  to  its  own  bulk.  (In 
Fig.  7,  the  space  occupied  by  the  cube  A  B  is  obviously 

equal  to  a  cube  of  water  of  the  same  size.)    The  FIG.  7. 

water  that  before  occupied  the  space  which  the 

body  now  fills  was  supported  by  the  pressure  of  the 

other  particles  of  water  around    it.     The  same 

pressure  is  exerted  on  the  substance  which  we 

have  immersed  in  the  water,  and,  consequently,  it 

will  be  supported  in  a  like  degree. 

When    will    a  If  the  b°dv  wei£ns  less  tlian  an 

body  sink,  and      equal  bulk  of  water,  the  pressure 

of  the  water  will  sustain  it  entirely, 

and  the  body  will  float ;  if,  on  the 
contrary,  it  is  heavier  than  an  equal  bulk  of  water, 
the  pressure  of  the  particles  of  water  will  be  un- 
able wholly  to  sustain  it,  and,  yielding  to  the  at- 
traction of  gravitation,  it  descends,  or  sinks. 

But  to  whatever  extent  a  body  may  be  supported  in  water,  to  the  same 
extent  it  will  cease  to  press  downward,  or  its  weight  will  diminish.  We  ac- 
cordingly find,  that  a  solid  body,  when  immersed  in  water  and  weighed,  will 
weigh  less  than  when  weighed  in  air,  and  the  difference  between  these  two 
weights  will  be  equal  to  the  weight  of  a  quantity  of  water  of  the  same  size  or 
bulk  as  the  solid  body ;  all  bodies  of  the  same  size,  therefore,  lose  the  same 
quantity  of  their  weight  in  water.  To  find  the  Specific  Gravity  of  Solids 
heavier  than  water,  or  their  weight  compared  with  the  weight  of  an  equal 
bulk  of  water,  we  have  the  following  rule : 

75.  Ascertain  the  weight  of  the  body  in 
water,  and  also  in  air.  Divide  the  weight  in 
air  by  the  loss  of  weight  in  water,  and  the 
quotient  will  be  the  specific  gravity  required. 


when  float,  in 
water  ? 


How  do  we  de- 
termine the 
Specific  Grav- 
ity of  Solids 
hearier  than 
water? 


40 


WELLS'S  NATURAL  PHILOSOPHY. 


FIG.  8. 


is  known. 

How  do  we 
find  the  Spe- 
cific Gravity 
of  a  body 
lighter  than 
water  ? 


Suppose  a  piece,  of  gold  weighs  in 
the  air  19  grains,  and  in  water  18 
grains  ;  the  loss  of  weight  in  water  will 
be  1;  19-r-l  =  19,  the  specific  gravity 
of  gold. 

Fig.  8  represents  the  arrangement  of 
the  balance  for  taking  specific  gravities, 
and  the  manner  of  suspending  the  body 
in  water  from  the  scale  pan,  or  beam, 
by  means  of  a  fine  thread,  or  hair. 

76.  To  find  the  specific 
gravity  of  a  body  lighter  than 
water,  tie  it  to  some  substance 
sufficiently  heavy  to  sink  it, 
whose  weight  in  air  and  water 
Weigh  the  two  together,  both  in  air  and  water, 
and  ascertain  the  loss  in  weight.  This  loss 
will  be  the  weight  of  as  much  water  as  is  equal 
in  bulk  to  the  two  solids  taken  together. 
Subtract  the  loss  of  the  heavy  body  weighed 
by  itself  in  water,  previously  known,  from  the  loss  sus- 
tained by  the  combined  solids.  The  remainder  will  be 
the  weight  of  as  much  water  as  is  equal  in  bulk  'to  the 
lighter  body.  Divide  the  weight  of  the  lighter  body  in 
air  by  this  remainder,  and  the  quotient  will  be  the  spe- 
cific gravity  required. 

*  Thus,  for  example,  let  the  weight  of  the  lighter  solid  be  3  ounces,  and  that 
of  the  heavier  solid  15  ounces.  Let  the  weight  which  the  two  together  lose 
when  submerged  in  water,  be  5  ounces,  and  let  the  weight  which  the  heavier 
alone  loses  when  immersed  be  1  ounce.  Subtracting  the  loss  of  weight  of  the 
heavier  body,  in  water,  1  ounce,  from  the  combined  loss  of  the  two  in  water, 
5  ounces,  we  have  4  ounces  as  the  weight  of  a  mass  of  water  equal  in  bulk  to 
the  lighter  body.  But  the  weight  of  the  lighter  body  in  air  is  3  ounces; 
3-*-4= 0.7  5 =£.  It  will,  therefore,  weigh  three  quarters  of  its  own  volume 
of  water,  or  have  a  specific  gravity  0.75. 

77.  The  specific  gravity  of  Liquids  may  also  be  found  by  ths 
Si°<T  theaysp£  balance  in  the  following  manner :  Weigh  a  solid  body  *n  water, 
cific  Gravity  as  well  as  in  the  liquid  whose  specific  gravity  is  to  be  de- 
balancl?bythe  termined ;  then  the  loss  in  each  case  will  be  the  respective 
weights  of  equal  bulks  of  water  and  liquid.  "We  have,  there- 
fore, the  following  rule : 

•78.  Divide  the  loss  of*  weight  in  the  liquid  by  the  loss 


SPECIFIC   GRAVITY,    OR   WEIGHT.  41 

of  weight  in  water  ;  the  quotient  will  give  the  specific 
gravity  of  the  liquid. 

Thus  a  solid  body  (a  piece  of  glass  is  generally  used)  loses  20  grains  when 
weighed  in  water,  and  30  grains  when  weighed  in  acid;  30-*-20  — 1.5,  the  spe- 
cific gravity  of  the  acid. 

79.  There  are  various  other  methods  of  obtaining  the  specific  gravity  of 
solids  and  liquids.*  Those  w.e  have  described  are  the  ones  most  generally 
adopted.  "/.  ''  +  '••.  .  . 

Hoirdoweob-  80.  For  obtaining  the  specific  gravity  of 
erne  '^rarity  gases,  air  instead  of  water  is  adopted  as  the 
of  a  Gas?  standard  of  comparison.  The  weight  of  a 
given  volume  or  measure  of  a  gas  is  compared  with  the 
weight  of  an  equal  volume  of  pure  atmospheric  air,  and 
the  weight  of  the  gas  divided  by  the  weight  of  the  air, 
will  express  the  specific  gravity  of  the  gas. 

81.  The  following  table  exhibits  the  specific  gravity  of  various  solid,  liquid, 
and  gaseous  bodies ;  pure  water,  having  a  temperature  of  60  degrees  Fahren- 
heit's thermometer,  being  assumed  as  the  standard  of  comparison  for  solids 
and  liquids,  and  pure,  dry  air,  having  the  same  temperature,  being  assumed 
as  the  standard  of  comparison  for  gases.  .  The  metal  platinum  has  the  greatest 
specific  gravity  of  any  solid  body,  being  21.50  times  heavier  than  an  equal 
bulk  of  water ;  and  hydrogen  gas  the  least  specific  gravity  of  any  of  the  gases, 
being  14.4  lighter  than  an  equal  bulk  of  air,  and  12.000  lighter  thanan  equal 
bulk  of  water.  These  two  substances  are  respectively  the  heaviest  and  light- 
est forms  of  matter  with  which  we  are  acquainted. 

SOLIDS  AND  LIQUIDS. 

Distilled  water 1.000 

Platinum • 21.500 

Gold 19.360 

Mercury 13.600 

Lead 11.450 

Silver .  10.500 

Copper 8.870 

Iron 7.800 

Flint  Glass 3.320 

Marble 2.830 

Anthracite  coal 1.800 

Box-wood 1.320 

Sea-water 1.020 

Whale  oil 0.920 

Pitch-pine  wood 0.660 

•  See  Hydrometer. 


42  WELLS'S  NATURAL  PHILOSOPHY. 

White  pine 0.420 

Alcohol. 0.800 

Ether 0.720 

Cork 0.240 

GASES. 

y    Pure,  dry  atmospheric  air 1.000 

Carbonic  acid  gas 1.520 

Oxygen.     .     .   *.    .    *, f •.*?.*?.  *r <•;•&£*,**•  •  i-100 

Nitrogen '^  •  «>^-.  .  0.970 

Ammoniacal  gas . 0.580 

Hydrogen 0.070 

How  can  we  82>  A  cubic  foot  of  water  weighs  almost  exactly  1,000 
determine  the  ounces  avoirdupois,  or  62|  pounds.  If,  therefore,  the  specific 
Sf^body  fr?m  S™*'1*?  of  water  be  represented  by  the  number  1,000,  the 
its  Specific  numbers  which  express  the  specific  gravity  of  all  other  solids 
and  liquids,  will  also  express  the  number  of  ounces  contained 
in  a  cubic  foot  of  their  dimensions.  Thus,  the  specific  gravity  of  gold  being 
19.360,  it  follows  that  a  cubic  foot  of  gold  will  weigh  19,360  ounces;  and  the 
specific  gravity  of  cork  being  0.240,  the  weight  of  a  cubic  foot  of  cork  will 
be  240  ounces.  By  means  of  a  table  of  specific  gravities,  therefore,  the 
weight  of  any  mass  of  matter  can  be  ascertained,  provided  we  know  its  cu- 
bical contents,  by  the  following  rule  : 

83.  Multiply  the  weight  of  a  cul)ic  foot  of  water  by 
the  specific  gravity  of  a  substance  ;  the  product  will  be 
the  weight  of  a  cubic  foot  of  that  substance. 

Thus,  anthracite  coal  has  a  specific  gravity  of  1.800.  This,  multiplied  by 
the  weight  of  a  cubic  foot  of  water,  1,000  ounces,  gives  1,800  ounces,  which 
is  the  weight  of  a  cubic  foot  of  coal. 

HOW  can  we  84.  The  volume,  or  bulk,  of  any  given  weight 
bulk"  oafiria  suhb8-  °f  a  substance  can  also  be  readily  calculated, 
ky  dividing  the  number  expressing  the  weight 
in  ounces  by  the  number  expressing  the  spe- 
cific gravity  of  the  substance,  omitting  the  decimal  points; 
the  quotient  will  express  the  number  of  cubic  feet  in  the 
volume,  or  bulk. 

Thus,  for  example,  if  it  be  desired  to  ascertain  the  bulk  of  a  ton  of  iron,  it 
is  only  necessary  to  reduce  the  ton  weight  to  ounces,  and  divide  the  number 
of  ounces  by  7.800,  the  specific  gravity  of  iron ;  the  quotient  will  be  the 
number  of  cubic  feet  in  the  ton  weight. 

of  matter  were  85.  If  the  particles  of  all  matter  were  per- 
hw  woiSd™1  fectly  free  to  move  among  themselves,  their 
themselves"80  arrangement  in  space  would  always  be  in  ex- 


SPECIFIC   GRAVITY,    OR   WEIGHT. 


43 


Why  does  a 
balloon  ascend, 
or  a  cork  rise 
to  the  surface 
of  water? 


act  accordance  with  their  different  specific  gravities  :  in 
other  words,  light  bodies,  or  those  having  a  small  specific 
gravity,  would  rest  upon,  or  rise  above  all  heavier  bodies, 
or  those  possessing  a  greater  specific  gravity. 

In  the  case  of  different  liquids,  the  particles  of  which  are 
What  are  illus- 
trations of  this  free  to  move  among  themselves,  this  arrangement  always  ex- 
principle?  jsj.gj  so  iong  as  the  different  substances  do  not  combine  to- 
gether, by  the  force  of  chemical  attraction,  to  form  a  compound  substance. 
Thus,  water  floats  upon  sulphuric  acid,  oil  upon  water,  and  alcohol  upon  oil, 
and  by  carefully  pouring  each  of  these  liquids  successively  upon  the  surface 
of  the  other,  they  may  be  arranged  in  a  glass  in  layers. 

Carbonic  acid  gas  is  heavier  than  atmospheric  air.     "We  accordingly  find 
that  it  accumulates  at  the  bottom  of  deep  pits,  wells,  caverns  and  mines. 

This  principle  also  explains  certain  phenomena  which  at 
first  seem  opposed  to  the  law  of  terrestrial  gravity,  that  all 
matter  is  attracted  toward  the  center  of  the  earth.  "We  ob- 
serve a  balloon,  a  soap-bubble,  or  a  cloud  of  smoke  or  steam 
to  ascend ;  and  a  cork,  or  other  light  body,  placed  at  the  bot- 
tom of  a  vessel  of  water,  rises  through  it,  and  swims  on  the  surface.  These 
phenomena  are  a  direct  consequence  of  gravitation ;  the  attraction  of  which, 
increasing  with  the  quantity  of  matter,  draws  down  the  denser  air  and  water 
to  occupy  the  place  filled  by  the  lighter  bodies,  which  are  thus  pushed  up, 
and  compelled  to  ascend. 

FIG.  9.  Suppose  or,  Fig.  9,  a  ball  of  wood  so  loaded  with  lead 

BMBBMBMEH.B  that  it  will  float  exactly  in  the  middle  of  a  vessel  of  water. 
The  weight  of  the  wood  and  the  upward  pressure  of  the 
water  have  such  a  relation  to  each  other,  that  the  ball  is 
balanced  in  this  position.  If  now  we  add  a  few  drops  of 
strong  salt  and  water,  we  shall  see,  as  it  sinks  and  mixes 
with  the  water,  that  the  ball,  a,  is  forced  to  the  top  of  the 
fluid,  because  the  attraction  of  gravitation  on  the  denser 
fluid  draws  it  down,  and  compels  it  to  occupy  the  place 
of  a. 

The  principle  that  the  particles  of  liquids  arrange  them- 
selves according  to  their  specific  gravities,  has  been  taken 
advantage  of  in  the  West  Indies  by  the  slaves,  in  order  to 
enable  them  to  steal  rum  from  casks.  The  long  neck  of  a  bottle  filled  with 
water,  is  inserted  through  the  bung  of  the  cask  into  the  rum.  The  water 
falls  out  of  the  bottle  into  the  cask,  while  the  lighter  rum  rises  to  take  its 
place. 

The  principle  of  specific  gravity  admits  of  many  valuable 

Mention  some  applications  in  the  arts.  It  offers  a  very  sure  and  quick 
of  the  practical  1F  J  . 

applications  of      method  of  determining  whether  a  substance  is  pure  or  adul- 
specific  grav-      terated.     Thus,  silver  may  be  mixed  with  gold  to  a  consider- 
able extent,  without  changing,  to  any  great  degree,  the  ap- 


44  WELLS'S  NATURAL  PHILOSOPHY. 

pearance  of  the  gold.  The  specific  gravity  of  pure  gold  being  19,  and  of  pure 
silver  10,  it  is  obvious  that  a  mixture  of  the  two  will  have  a  specific  gravity 
less  than  pure  gold,  and  greater  than  pure  silver,  the  difference  being  propor- 
tioned to  the  amount  of  adulteration.  In  the  same  way  we  can  determine 
whether  cheap  oils  have  been  mixed  with  expensive  oils,  cheap  and  poor  il- 
luminating gas,  with  expensive  and  brilliant  gas.  In  any  case  it  enables  us 
to  ascertain  the  exact  size  or  solid  bulk  of  a  mass,  however  irregular— even 
of  a  bundle  of  twigs.* 

€&Cr-'id 

PRACTICAL    PROBLEMS   RELATING   TO    SPECIFIC    GRAVITY. 

« 

1.  The  weight  of  a  solid  body  is  200  grains,  but  its  weight  in  water  is  only  150  grams ; 
what  is  the  specific  gravity  of  the  body  ? 

Solution:  50  grains  =  loss  of  weight  in  water;  200  grains  (weight  in  air)-?- 50 =4,  spe- 
cific gravity  required. 

2.  A  body  weighed  in  the  air  28  pounds,  and  in  water  24  pounds ;  what  is  its  specific 
gravity  ? 

3.  An  irregular  fragment  of  stone  weighed  in  air  78  grains,  but  lost  30  upon  being 
weighed  in  water;  what  was  the  specific  gravity  of  the  stone? 

4.  A  piece  of  cork  weighed  in  the  air  48  grains,  and  a  piece  of  brass  560  grains ;  the 
brass  weighed  in  water  4S8  grains,  and  the  brass  and  cork  when  tied  together  weighed  in 
water  336  grains.    What  was  the  specific  gravity  of  the  cork  ? 

5.  How  much  more  matter  is  there  in  a  cubic  foot  of  sea-water,  than  in  a  cubic  foot  of 
fresh  water  ? 

6.  Would  a  piece  of  steel  sink  or  swim  in  melted  copper  ? 

7.  When  alcohol  and  whale-oil  are  put  in  the  same  vessel,  which  of  these  two  sub- 
stances will  occupy  the  top,  and  which  the  bottom  part  of  the  vessel? 

8.  If  a  cubic  foot  of  water  weigh  1,000  ounces,  what  will  be  the  weight  of  a  cubic  foot 
of  lead? 

9.  What  will  be  the  weight  of  a  cubic  foot  of  cork,  in  ounces  and  in  pounds  ? 

*  The  attempt  to  ascertain  whether  a  particular  body  had  been  adulterated  led  Archi- 
medes, it  is  said,  to  the  discovery  of  the  principle  of  specific  gravity.  Hiero,  King  of 
Syracuse,  having  bought  a  crown  of  gold,  desired  to  know  if  it  were  formed  of  pure  metal ; 
and  as  the  workmanship  was  costly,  he  wished  to  accomplish  this  without  defacing  it. 
The  problem  was  referred  to  Archimedes.  The  philosopher  for  some  time  was  unable  to 
solve  it,  but  being  in  the  bath  one  day,  he  observed  that  the  water  rose  in  the  bath  in  ex- 
act proportion  to  the  bulk  of  his  body  beneath  the  surface  of  the  water.  He  instantly  per- 
ceived that  any  other  substance  of  equal  sizs,  would  raise  the  water  just  as  much,  though 
one  of  equal  weight  and  less  size,  or  bulk,  could  not  produce  the  same  effect  Convinced 
that  he  could,  by  the  application  of  this  principle,  determine  whether  Hiero's  crown  had 
been  adulterated,  and  moved  with  admiration  and  delight,  he  is  said  to  have  leaped  from 
the  water  and  rushed  naked  into  the  street,  crying  "  Svp^a  I  Evpnxa  1"  "  I  have  found  it! 
I  have  found  it !"  In  order  to  apply  his  theory  to  practice,  he  procured  a  mass  of  pure  gold 
and  another  of  pure  silver,  each  having  the  same  weight  as  the  crown  ;  then  plunging  the 
three  metallic  bodies  successively  into  a  vessel  quite  filled  with  water,  and  having  carefully 
collected  and  weighed  the  quantity  of  liquid  which  was  displaced  in  each  instance,  h« 
ascertained  that  the  mass  of  pure  gold,  of  the  same  weight  as  the  crown,  displaced  less 
water  than  the  crown  ;  the  crown  was,  therefore,  not  pure  gold.  The  mass  of  pure  silver 
of  the  same  weight  as  the  crown,  displaced  more  waterthan  the  crown;  the  crown,  there- 
fore, was  not  pure  silver,  but  a  mixture  of  gold  and  silver. 


CENTER   OF    GRAVITY  45 

10.  How  many  cubic  feet  in  a  ton  of  gold  ? 

11.  How  many  cubic  feet  in  two  tons  of  anthracite  coal  ? 

12.  How  many  cubic  feet  in  a  ton  of  cork  ? 

13.  A  fragment  of  metal  lost  5  ounces  when  weighed  in  water ;  what  were  its  dimen- 
sions, supposing  a  cubic  foot  of  water  to  weigh  1,000  ounces  ? 

Solution :  The  loss  of  weight  in  water,  5  ounces,  is  the  weight  of  a  bulk  of  water  equal 
to  that  of  the  body.  As  we  know  the  weight  of  a  cubic  foot  of  water,  we  can  determine 
the  number  of  cubic  inches  or  feet  in  any  given  weight,  thus ;  as  1,000  (the  weight  of  a  cubic 
foot  of  water  in  ounces)  is  to  5  ounces,  so  is  1,728  (the  number  of  cubic  inches  in  a  cubic 
foot)  to  8.64  cubic  inches,  the  dimensions  of  the  fragment. 

14.  Wishing  to  ascertain  the  number  of  cubic  inches  in  an  irregular  fragment  of  stone, 
it  was  weighed  in  water,  and  its  loss  of  weight  observed  to  be  4.25  ounces.    What  were  its 
dimensions  ? 

SECTION    III. 

CENTER     OF     GRAVITY. 

86.  The  CENTER  of  GRAVITY  in  a  body,  is 

What    is    the  .  11 

center  of  Grav-    that   point   about   which,   if   supported,    the 

ity  in  a  body  ?  *  '  *  r 

whole  body  will  balance  itself. 

FlG  1Q  If  we  take  a  rod,  or  beam,  of 

equal  size  throughout,  and  suspend 
it  from  the  middle,  Fig.  10,  the 
two  sides  will  exactly  balance  each 

'  other,  and  it  will  remain  at  rest  in 

a  horizontal  position.     There  being 

as  much  matter  similarly  situated  on  one  side  of  the  support  as  on  the  other, 
the  force  of  attraction  exerted  on  both  sides  will  be  alike,  and  therefore  one 
side  can  not  overpower,  or  outweigh  the  other. 

In  every  body,  of  whatever  size  or  form,  a  point  may  be 
collide?7  the  found>  about  which,  if  supported,  all  the  parts  of  the  body  will 
whole  attrac-  balance,  or  remain  at  rest.  Everybody  may  be  considered  as 
a  bodXeconcen"  ma(le  up  of  separate  particles,  each  acted  upon  separately  by 
truted  at  its  gravity,  but  as  by  supporting  this  one  point  we  support  the 
Center  of  Grav-  whol6j  ag  by  liftmg  it  we  lift  the  whole,  and  as  by  stopping  it 

we  cause  the  whole  body  to  rest,  the  whole  attraction  exerted 
on  the  entire  mass  may  be  considered  as  concentrated  at  this  one  point,  and 
this  point  we  call  the  CENTER  of  GRAVITY. 

87.  The  CENTER  OF  MAGNITUDE  of  a  body, 

What  is  the  t/7 

center  of  Mag-     is  the  central  point  of  the  bulk,  or  mass  of  the 

nitude  ?  * 

body. 

88.  When  a  body  is  of  uniform  density,  the 

Where    is    the  J  .   -,       . 

Center  of  Grav-     CENTER     OF     GRAVITY    Will     Coincide    With    its 

ity  of  a  body  ? 

center  of  magnitude  ;  but  when  one  part  01  a 
body  is  composed  of  heavier  materials  than  another  part, 


46  WELLS'S  NATURAL   PHILOSOPHY. 

the  center  of  gravity  no  longer  corresponds  with,  the  center 
of  magnitude,  or  the  central  point  of  the  hulk  of  the  hody. 

FIG.  11.  Thus,  in  a  sphere,  a  cube,  or  a  cylinder,  the  center  of  grav- 

ity is  the  same  as  the  center  of  the  body.  In  a  ring  of  uni- 
form size  and  density,  the  center  of  gravity  is  the  center  of 
the  space  inclosed  in  the  ring  (see  Fig.  11).  This  example 
shows  that  the  center  of  gravity  is  not  necessarily  included 
in  that  portion  of  space  occupied  by  the  matter  of  the  body. 

In  a  wheel  of  wood  of  uniform  density  and  thickness  the 
center  of  gravity  will  be  the  center  of  the  wheel,  but  if  a  part  of  the  rim  be 
made  of  iron,  the  center  of  gravity  will  be  removed  to  some  point  aside  from 
the  center. 

"When  two  bodies  are  connected  together,  they  may  be  regarded  as  one 
body,  having  but  one  center  of  gravity.  If  the  two  bodies  be  of  equal  weight, 
the  center  of  gravity  will  be  in  the  middle  of  the  line  which  unites  them ; 
but  if  one  be  heavier  than  the  other,  the  center  of  gravity  will  be  as  much 
nearer  the  heavier  body,  as  the  heavier  exceeds  the  lighter  one  in  weight. 
f!Qt  12.  Thus,  if  two  balls,  each  weighing  four  pounds,  be 

j  connected  together  by  a  bar,  the  center  of  gravity 

will  be  a  point  on  the  bar  equally  distant  from 
each.  But  if  one  of  the  balls  be  heavier  than  the 
other,  then  the  center  of  gravity  will,  in  propor- 
tion, approach  the  larger  ball.  This  is  illustrated  by  reference  to  Fig.  12,  in 
which  the  center  of  gravity  about  which  the  two  balls  support  themselves,  is 
seen  to  be  nearest  to  the  heavier  and  larger  ball. 

89.  The  center  of  gravity  of  a  body  being  regarded  as  the 
Center  ofGrav-  Pomt  m  which  the  sum  of  all  the  forces  of  gravity  acting  upon 
ity  be  in  penna-  the  separate  particles  of  the  body  are  concentrated,  it  may 
equilibrium  ?  °r  ^e  considered  as  influenced  by  the  attraction  of  the  earth 
in  a  greater  degree  than  any  other  portion  of  the  body.  It 
follows,  therefore,  that  if  a  body  has  freedom  of  motion,  it  can  not  be  brought 
into  a  position  of  permanent  equilibrium,  until  its  center  of  gravity  occupies 
the  lowest  situation  which  the  support  of  the  body  will  allow ;  that  is,  the 
center  of  gravity  will  descend  as  far  toward  the  center  of  the  earth  as  possible. 

90.  By  EQUILIBRIUM  we  mean  a  state  of  rest 

What    do    we  J  . 

17  Equili"   Pr°duced  "7  the  counterpoise,  or  balancing,  of 
opposite  forces. 

Thus  when  one  force  tending  to  produce  motion  in  one  direction,  is  opposed 
by  an  equal  force  tending  to  produce  motion  in  an  exactly  opposite  direction, 
the  two  balance  each  other,  and  no  motion  results.  To  produce  any  action, 
there  must  be  an  inequality  in  the  condition  of  one  of  the  forces. 

The  truth  of  this  principle  may  be  illustrated  by  certain  ex- 
periment  can     periments  which  at  first  seem  to  be  contradictory  to  it.     Thus 

this  *        a  cylinder  may  be  made  to  ro11  UP  an  inclined  Plane-     Fix  a 

piece  of  lead,  I,  Fig.  13,  on  one  side  of  the  cylinder  a,  so  that 


CENTER    OF    GRAVITY. 


47 


FIG.  13. 


In  what  three 
ways  may  the 
Center  of  GraY- 
ity  be  support- 
ed? 


Illustrate 
first  case. 


the 


the  center  of  gravity  of  the  cylinder  will  be  at  the  point  Z,  while  its  center 

of  magnitude  is  at  c.  The  cylinder 
will  then  roll  up  the  inclined  plane  to 
the  position  a  /,  because  the  center 
of  gravity  of  the  mass,  I,  will  endeavor 
to  descend  to  its  lowest  point. 

91.  A  prop  that  supports 
the  center  of  gravity  sup- 
ports the  whole  body.     This  support  may  be  applied  in 
three  different  ways  : 

1.  The  point  of  support  may  be  applied  di- 
rectly to  the  center  of  gravity  of  the  body. 

2.  The  point  of  support  may  have  the  cen- 
ter of  gravity  immediately  below  it. 

3.  The  point  of  support  may  have  the  center  of  gravity 
immediately  above  it. 

In  the  first  case,  where  the  point  of  support  is  applied  di- 
rectVy  to  the  center  of  gravity,  the  body  will  remain  at  rest  in 
any  position ;  this  is  illustrated  in  the  case  of  a  common  wheel, 
where  the  center  of  gravity  is  also  the  center  of  the  figure,  and  this  being 

supported  on  the  axle,  the  wheel  rests 
indifferently  in  any  position.  In  Fig. 
14,  let  a,  the  center  of  the  wheel,  which 
is  also  its  center  of  gravity,  be  supported 
by  an  axle ; — the  wheel  rests,  no  matter 
•a-  to  what  extent  we  turn  it. 

In  the  second  case,  where  the  point 
•C  of  support  is  above  the  center  of  gravity, 
the  body,  if  it  is  allowed  freedom  of  mo- 
tion, will  not  rest  in  perfect  equilibrio 
until  its  center  of  gravity  has  descended  to  the  lowest  position,  which  in  all 
cases  will  be  immediately  beneath  the  point  of  suspension. 
Thus,  in  Fig.  14,  let  the  wheel,  the  center  of  gravity  of  which 
is  at  a,  be  suspended  from  the  point  &,  by  a  thread,  or  hung 
upon  an  axle,  having  freedom  of  motion  on  that  point.  However  much  we 
may  move  it,  either  right  or  left,  toward  m  or  n,  as  shown  by  the  dotted  lines, 
am  and  an,  it  swings  back  again,  and  is  only  at  rest  when  b  and  a  are  in  the 
same  perpendicular  line. 

In  the  third  case,  where  the  point  of  support  has  the  een- 
thini  cas^. th<3      'er  °^  grayity  above  it,  a  body  will  remain  at  rest  only  so  long 
as  the  center  of  gravity  is  in  a  vertical 'line,  above  the  point 
of  support.     In  Fig.  14,  suppose  the  wheel  to  be  supported  at  the  point  c,  sit- 
uated in  a  vertical  line  a  c,  immediately  below  the  center  of  gravity,  a;  so 


FIG.  14, 


48  WELLS'S   NATURAL   PHILOSOPHY. 

long  as  this  position  is  maintained,  the  wheel  will  remain  at  rest,  but  the  mo- 
ment the  center  of  gravity,  a,  is  moved  a  little  to  the  right  or  left,  so  as  to 
throw  it  out  of  the  vertical  line  joining  a  and  c,  the  wheel  will  turn  over,  and 
assume  such  a  position  as  to  bring  the  center  of  gravity  immediately  beneath 
the  point  of  support,  as  in  the  second  case. 

upon  what  92.  The  stability  of  a  body,  therefore,  de- 

bfittyXlbodJ  pends  upon  the  manner  in  which  it  is  sup- 
depend?  ported,  or  in  other  words,  upon  the  position 

of  its  center  of  gravity. 

what  are  the  93.  As  a  body  may  be  supported  in  three 
positions,  we  have,  as  a  consequence,  three 
conditions  of  equilibrium,  viz.,  Indifferent, 
Stable,  and  Unstable  Equilibrium. 

What  is  indif  INDIFFERENT  EQUILIBRIUM  occurs  when  a  body  is  supported 

ferent  Equili-  upon  its  center  of  gravity  ;  for  then  it  remains  at  rest  indiffer- 
brium?  ently  in  eyerv  pOSition> 

STABLE  EQUILIBRIUM  occurs  when  the  point  of  support  is 
Equilibrium  ?°      above  the  center  of  gravity.     If  a  body  be  moved  from  this 
position,   it  swings  backward  and  forward  for  a  time,  and 
finally  returns  to  its  original  situation. 

What  is  Un  UNSTABLE  EQUILIBRIUM  occurs  when  the  point  of  support  is 

stable  Equili-  beneath  the  center  of  gravity.  The  tendency  of  the  center  of 
brium  ?  gravity  in  such  cases  is  to  change,  and  take  the  lowest  situation 

the  support  of  the  body  will  allow. 

94.  The  principle  that  when  a  body  is  suspended  freely,  it 
the      w^l  have  its  center  of  gravity  in  a  vertical  line,  immediately 


center  of  gray-      below  the  point  of  support,  ha.s  been  taken  advantage  of  to 
bodies?  °  determine  experimentally  the  position  of  the  certer  of  gravity, 

in  irregular  shaped  bodies.     Suppose  we  suspend,  as  in  Fig-. 
15,  an  irregular  piece  of  board  by  means  of  cord.     A  plumb-line  let  fall  frora 
....  the  point  of  support,  or  the  prolongation  of  the  cord,  w/.ll 

pass  through  the  center  of  gravity,  Gr.  If  we  now  attach 
the  cord  to  another  point,  and  suspend  the  body  anew,  the 
prolongation  of  the  cord  in  this  instance,  also,  will  pasa 
through  the  center  of  gravity,  Gr.  The  intersection  of 
these  two  lines  will  be  the  center  of  gravity,  and  the 
board,  if  suspended  by  a  cord  attached  to  this  point,  will 
hang  evenly  balanced. 

95.  A  line  which  connects  the  center  of 
gravity  of  a  body  with  the  center  of  the 
earth,  or,  in  other  words,  a  line  drawn  from 
the  center  of  gravity  perpendicularly  downward,  is  called 
the  .LINE  of  DIRECTION.  It  is  called  the  Line  of  Direction, 


CENTER   OF   GRAVITY. 


49 


what  is  the    because  when  a  solid  body  falls,  its  center  of 

LineofDirec-  .        *.  '  . 

tion?  gravity  moves  along  this  line  until  it  reaches 

the  ground.  When  bodies  are  supported  upon  a  basis, 
their  stability  depends  on  the  position  of  their  Line  of 
Direction. 

96.  If  the  line  of  direction  falls  within  the 
^ase  uPon  which  tne  body  stands,  the  body 
remains  supported  ;  but  if  it  falls  without  the 
base,  the  body  overturns. 

FIG.  16,  FIG.  17. 


when  wm  a 

andywheanwiii 
it  fan? 


FIG.  18. 


Thus,  in  Fig.  16,  the  line  directed  vertically  from  the  center  of  gravity,  G-, 
falls  within  the  base  of  the  body,  and  it  remains  standing;  but  in  Fig.  17  a 
similar  line  falls  without  the  base,  and  the  body,  consequently,  can  not  be 
maintained  in  an  upright  position,  and  must  fall. 

A  wall,  or  tower  stands  securely,  so  long  as  the  perpendicular  line  drawn 

through  its  center  of  gravity  falls 
within  its  base.  The  celebrated 
leaning-tower  of  Pisa,  315  feet  high, 
which  inclines  12  feet  from  a  per- 
fectly upright  position,  is  an  example 
of  this  principle.  For  instance,  the 
line  in  Fig.  18,  falling  from  the  top 
of  the  tower  to  the  ground,  and 
passing  through  the  center  of  gravity, 
falls  within  the  base,  and  the  tower 
stands  securely.  If,  however,  an 
attempt  had  been  made  to  build  the 
tower  a  little  higher,  so  that  the  per- 
pendicular line  passing  through  the 
center  of  gravity,  would  have  fallen 
beyond  the  base,  the  structure  cculd 
no  longer  have  supported  itself 

97.  The  broader,  or  larger 


50 


WELLS'S   NATURAL   PHILOSOPHY. 


the  base  of  a  body,  and  the  nearer  its  principal  mass  is  to 
the  base,  or,  in  other  words,  the  lower  its  cen- 
ter of  gravity  is,  the  firmer  it  will  stand. 

A  pyramid,  for  this  reason,  is  the  firmest  of  all  structures. 
The  base  upon  which  the  human  body  rests,  or  is  supported, 
is  the  two  feet  and  the  space  included  between  them.  The 
advantage  of  turning  out  the  toes  when  we  walk  is,  that  it 
increases  the  breadth  of  the  base  supporting  the  body,  and 
enables  us  to  stand  more  securely. 

In  every  movement  of  the  body,  a  man  adjusts  his  position  unconsciously, 
in  such  a  way  as  to  support  the  center  of  gravity,  and  cause  the  line  of  di- 
rection to  fall  within  the  base. 

Why  does  a  j^  person  carrying  a  load  upon  his  back,  bends  forward  in 
order  to  bring  the  center  of  gravity  and  his  load  over  his 


When    will    a 
body      stand 
most  firmly  ? 


What  is  the 
advantage  of 
turning  out  the 
toes  in  walk- 
ing? 


on    his     back 
bend  over? 


FIG.  19. 


Fm.  20. 


Why  does  a 
person  lean  for- 
ward in  ascend- 
ing a  hill,  and 
backward  in 
descending  ? 


Why  is  a  high 
carriage  more 
liable  to  over- 
turn than  a  low- 
one? 


FIG.  21. 


If  he  carried  the  load  in  the  position  of  A,  Fig.  19,  he  would  be  liable  to 
fall  backward,  as  the  direction  of  the  center  of  gravity  would  fall  beyond  his 
heels ;  to  bring  the  center  of  gravity  over  his  feet,  he  assumes  the  position 
indicated  by  B,  Fig.  20. 

For  the  same  reason,  when  a 
man  ascends  a  hill  he  leans  for- 
ward, and  when  he  descends  he 
leans  backward.  See  Fig.  21. 

A  high  carriage  is  much  more 
liable  to  be  overset  by  an  irregu- 
larity in  the  road  than  a  low  one ; 
because  the  center  of  gravity  being 
high,  the  line  of  direction  is  easily 
thrown  without  the  base.  This 
will  appear  evident  from  the  following  illustration,  Fig.  22. 


CENTER  OF   GRAVITY. 


FiG.  23. 


Let  A  represent  a  coach  standing  on  a  level ;  B,  a  cart  loaded  -with  stones 
on  a  slope ;  C,  a  wagon  loaded  with  hay  on  a  slope ;  a  a  a  the  centers  of 
gravity ;  a  6,  line  of  direction ;  c  cf,  base. 

.  Here  it  is  obvious  that  the  hay-wagon  must  upset,  because  the  line  of  di- 
rection falls  without  the  base ;  that  the  coach  is  very  secure,  because  the  line 
of  direction  falls  far  within  the  base ;  and  the  stone-cart,  though  the  center 
of  gravity  is  low  down,  is  not  very  secure,  because  the  line  of  direction  falls 
very  near  the  outside  of  the  base. 

The  effect  on  the  stability  of  a  body  occa- 
sioned by  placing  its  center  of  gravity  in  a  very 
low  position,  is  shown  in  an  amusing  toy  for 
children,  represented  by  Fig.  23.  The  horse, 
with  his  rider,  is  firmly  supported  on  his  hind 
feet,  because,  by  means  of  a  leaden  ball  attached 
to  the  bent  wire,  the  center  of  gravity  is  brought 
below  the  point  of  support. 
^^  If  a  body  be  placed  on  an  in- 

body  slide  and  clined  surface,  it  will  slide  down 
when  its  line  of  direction  falls 
within  the  base  ;  but  it  will  roll 


FIG.  24. 


down  when  it  falls  with- 
out the  base.  Thus  the 
body,  e,  Fig.  24,  having  its  line  of  direction  e  a,  with- 
in the  base,  will  slide  down  the  inclined  surface,  c  d; 
but  the  body  &  a,  will  roll  down,  since  its  line  of  di- 
rection, 6  a,  falls  without  the  base. 


PRACTICAL   QUESTIONS  ON  THE   CENTER   OF  GRAVITY. 

1.  Why  does  a  person  in  rising  from  a  chair  bend  forward  ? 

"When  a  person  is  sitting,  the  center  of  gravity  is  supported  by  the  seat ; 
in  an  erect  position,  the  center  of  gravity  is  supported  by  the  feet;  therefore, 
before  rising  it  is  necessary  to  change  the  center  of  gravity,  and,  by  bending 
forward,  we  transfer  it  from  the  chair  to  a  point  over  the  feet. 


52  WELLS'S  NATURAL  PHILOSOPHY. 

2.  Why  is  a  turtle  placed  on  its  back  unable  to  move  ? 

Because  the  center  of  gravity  of  the  turtle  is,  in  this  position,  at  the  loioest 
point,  and  the  animal  is  unable  to  change  it  j  therefore  it  is  obliged  to  remain 
at  rest. 

3.  Why  do  very  fat  people  throw  back  their  head  and  shoulders  when  they  walk  ? 

In  order  that  they  may  effectually  keep  the  center  of  gravity  of  the  body 
over  the  base  formed  by  the  soles  of  the  feet. 

4.  Why  can  not  a  man,  standing  with  his  heels  close  to  a  perpendicular  wall,  bend  over 
sufficiently  to  pick  up  any  object  that  lies  before  him  ou  the  ground,  without  falling? 

Because  the  wall  prevents  him  from  throwing  part  of  his  body  backward, 
to  counterbalance  the  head  and  arms  that  must  project  forward. 

5.  What  is  the  reason  that  persons  walking  arm-in-arm  shake  and  jostle  each  other, 
unless  they  make  the  movements  of  their  feet  to  correspond,  as  soldiers  do  in  marching? 

"When  we  walk  at  a  moderate  rate,  the  center  of  gravity  comes  alternately 
over  the  right  and  over  the  left  foot.  The  body  advances,  therefore,  in  a  wav- 
ing line ;  and  unless  two  persons  walking  together  keep  step,  the  waving  mo- 
tion of  the  two  fails  to  coincide. 

6.  In  what  does  the  art  of  balancing  or  walking  upon  a  rope  consist? 

In  keeping  the  center  of  gravity  in  a  line  over  the  base  upon  which  the 
body  rests. 

7.  Why  is  it  a  very  difficult  thing  for  children  to  learn  to  walk  ? 

In  consequence  of  the  natural  upright  position  of  the  human  body,  it  is 
constantly  necessary  to  employ  some  exertion  to  keep  our  balance,  or  to  pre- 
vent ourselves  from  falling,  when  we  place  one  foot  before  the  other.  Chil- 
dren, after  they  acquire  strength  to  stand,  are  obliged  to  acquire  this  knowl- 
edge of  preserving  the  balance  by  experience.  When  the  art  is  once  ac- 
quired, the  necessary  actions  are  performed  involuntarily. 

8.  Why  do  young  quadrupeds  learn  to  walk  much  sooner  than  children  ? 
Because  a  body  is  tottering  in  proportion  to  its  great  altitude  and  narrow 

lose.  A  child  has  a  body  thus  constituted,  and  learns  to  walk  but  slowly  be- 
cause of  this  difficulty  (perhaps  in  ten  or  twelve  months),  while  the  young  of 
quadrupeds,  having  a  broad  supporting  base,  are  able  to  stand  and  move  about 
almost  immediately. 

9.  Are  all  the  limbs  of  a  tall  tree  arranged  in  such  a  manner,  that  the  line  directed 
from  the  center  of  gravity  is  caused  to  fall  within  the  base  of  the  tree  ? 

Nature  causes  the  various  limbs  to  shoot  out  and  grow  from  the  sides' 
with  as  much  exactness,  in  respect  of  keeping  the  center  of  gravity  within 
the  base,  as  though  they  had  been  all  arranged  artificially.  Each  limb  grpws, 
in  respect  to  all  the  others,  in  such  a  manner  as  to  preserve  a  due  balance  be- 
tween the  whole. 


LAWS   OF   FALLING  BODIES.  53 

SECTION    IV. 

EFFECTS  OF  GRAVITY  AS  DISPLAYED  BY  FALLING  BODIEa 

98.  When  an  unsupported  body  falls,  its 
motion  will  be  in  a  straight  line  toward  the 
center  of  the  earth.  This  line  is  called  a  VERTICAL 
LINE. 

what  is  a  99.  If  a  body  be  suspended  by  a  thread,  the 

piumb  Line?  thread  will  always  assume  a  vertical  direction, 
or  it  will  represent  that  path  in  which  the  body  would 
have  fallen.  A  weight  thus  suspended  by 
a  thread,  is  called  a  PLUMB-LINE,*  Fig.  25, 
and  is  used  by  carpenters,  masons,  etc.,  to 
ascertain  by  comparison,  whether  their  work 
stands  in  a  vertical  or  perpendicular  position, 
what  is  a  100.  A  plumb-line  is  always 

Level  surface?  perpendicular  to  the  surface  of 
water  at  rest.  The  position  of  such  a  sur- 
face we  call  LEVEL. 

No  two  plumb-lines  upon  the  earth's  surface  will  be 
parallel,  but  will  incline  toward  each  other,  since  no  two 
bodies  from  different  points  can  approach  the  center  of  a 
sphere  in  a  parallel  direction.  If  their  distance  apart  be 

one  mile,  this  inclination  will  amount  to  one  minute,  FIG.  26. 

and  if  it  be  sixty  miles,  to  one  degree.     In  Fig.  26,  W 

let  E  E  be  a  portion  of  the  earth's  surface,  and  D  its  * 

center;  the  bodies  A,  B,  and  C,  when  allowed  to 

drop,  will  fall  hi  the  direction  A  D,  B  D,  and  C  D. 

wai  an  bodies,        101.  As  the  attraction  of  E- 
•fluenceof!Uv~-    tne  earth  acts  equally  and 
wlthalequkiv"    independently   on    all    the 
lodties?  particles  composing  a  body, 

it  is  clear  that  they  must  all  fall  with 
equal  velocities.  It  makes  no  difference  whether  the  sev- 
eral particles  fall  singly,  or  whether  they  fall  compacted 
together,  in  the  form  of  a  large  or  a  small  body. 

i   *  Plumb  Line,  so  called  from  the  Latin  word  plumbum,  lead,  the  weight  nsually  at- 
tached to  the  string. 


WELLS'S  NATURAL  PHILOSOPHY. 


Fio.  27. 


If  ten  or  a  hundred  leaden  balls  be  disengaged  together,  they  will  fall  in 
the  same  time,  and  if  they  be  molded  into  one  ball  of  great  magnitude,  it 
will  still  fall  in  the  same  manner. 

102.  Hence  all  bodies  under  the  influence  of  gravity- 
alone,  must  fall  with  equal  velocities.* 

B     what  er  There  are  some  familiar  facts  which  seem 

periment  can  to  be  opposed  to  this  law.  "When  we  let  go 
you  prove  this  a  feather  and  a  mass  of  lead,  the  one  floats 
in  the  air,  and  the  other  falls  to  the  ground  very 
rapidly.  But  hi  this  case,  the  operation  of  gravity  is  modified 
by  the  resistance  of  the  air ;  the  feather  floats  because  the 
air  opposes  its  descent,  and  it  can  not  overcome  the  resistance 
offered.  But  if  we  place  a  mass  of  lead  and  a  feather  in  a 
vessel  exhausted  of  air,  and  liberate  them  at  the  same  time, 
they  will  fall  in  equal  periods.  The  experiment  is  easily 
shown  by  taking  a  glass  tube,  Fig.  27,  closed  at  one  end,  and 
supplied  with  an  air-tight  cap  and  screw-cock  at  the  other. 
A  feather  and  a  piece  of  metal  are  previously  inclosed  in  the 
tube.  The  tube  being  filled  with  air,  and  inverted,  the  metal 
will  fall  with  greater  speed  than  the  feather,  as  might  be  ex- 
pected. If  the  tube  be  now  exhausted  of  air  by  means  of 
an  air-pump  and  the  screw-cock,  and  in  this  condition  in- 
verted, the  feather  and  the  metal  will  fall  from  end  to  end 
of  the  tube  with  equal  velocity. 

103.  If  a  man  leap  from  a  chair  or  table, 
he  will  strike  the  ground  without  injury.  .  If 
the  same  man  leap  from  the  top  of  a  high 
house,  he  will  probably  be  killed.  These, 
and  many  like  instances,  prove  that  the  force 
with  which  a  falling  body  strikes  the  ground  depends  upon 
the  height  from  which  it  falls.  But  the  force  depends  on 
the  velocity  of  the  body  the  moment  it  touches  the  ground; 
therefore,  the  velocity  with  which  a  body  falls  depends  also 
upon  the  height  from  which  it  descends. 


Upon  what  do 
the  force  and 
velocities  of 
falling  bodies 
depend? 


*  Previous  to  the  time  of  Galileo,  the  philosophers  maintained  that  the  velocity  of  a 
falling  body  was  in  proportion  to  its  weight,  and  that  if  two  bodies  of  unequal  weights, 
were  let  fall  from  an  elevation,  at  the  same  moment,  the  heavier  would  reach  the  ground 
as  much  sooner  than  the  lighter,  as  its  weight  exceeded  it.  In  other  words,  a  body  weigh- 
ing two  pounds  would  fall  in  half  the  time  that  would  be  required  by  a  body  weighing  one 
pound.  Galileo,  on  the  contrary,  asserted  that  the  velocity  of  a  falling  body  is  independant 
of  its  weight,  and  not  affected  by  it.  The  dispute  running  high,  and  the  opinion  of  the 
public  being  generally  averse  to  the  views  of  Galileo,  he  challenged  his  opponents  to  test 
the  matter  by  a  public  experiment.  The  challenge  was  accepted,  and  the  celebrated  leaning- 
tower  of  Pisa  agreed  upon  as  the  place  of  trial.  In  the  presence  of  a  large  concourse,  two 
balls  were  selected,  one  having  exactly  twice  the  weight  of  the  other.  The  two  were  then 
dropped  from  the  summit  of  the  tower  at  the  same  moment,  and,  in  exact  accordance 
with  the  assertions  of  Galileo,  they  both  struck  the  ground  at  the  same  instant 


LAWS   OF   FALLING   BODIES.  55 

How  does  104.  "When  a  body  falls,  it  is  attracted  by  gravity  during 

f  fainv*  body™  tlie  wnole  time  of  its  Ailing.  Gravity  does  not  merely  set 
the  body  in  motion  and  then  cease,  but  it  continues  to  act. 
During  the  first  second  of  time,  the  force  of  gravity  will  cause  the  body  to 
'descend  through  a  certain  space.  At  the  end  of  this  time,  the  body  would 
continue  to  move,  with  the  motion  it  has  acquired,  without  the  action  of  any 
further  force,  merely  on  account  of  its  inertia.  But  gravity  continues  to  act, 
and  will  add  as  much  more  motion  to  the  falling  body  during  the  second 
second  of  time,  as  it  did  during  the  first  second,  and  as  much  again  during 
the  third  second,  and  so  on. 

what  is  the  "^'  Calling  bodies,  therefore,  descend  to 
faUipg  ^Q  earth  with  a  uniform  accelerated  motion. 
A  body  falling  from  a  height  will  fall  16  feet 
in  the  first  second  of  time,*  three  times  that  distance  in 
the  second,  five  times  in  the  third,  seven  in  the  fourth, 
the  spaces  passed  over  in  each  second  increasing  as  the 
odd  numbers  1,  3,  5,  7,  9,  11,  etc. 

"ROW  does  the  106.  The  entire  space  passed  over  by  a  body 
over6  anTfhe  in  falling  is  as  the  square  of  the  time  ;  that  is, 
Jngebodyacoam~-  ™  twice  the  time  it  will  fall  through  four  times 
Pare?  the  space  ;  in  thrice  the  time,  nine  times  the 

space,  f 

The  time  occupied  in  falling,  therefore,  being  known,  the  height  from  which 
a  body  falls  may  be  calculated  by  the  following  rule  : 

Time  bein~  1^'  Multiply  the  square  of  the  number  of 

^vln'-huVan     seconds  of  time  consumed  in  falling,  by  the 

theheightfrom          .  .  MI  /•  11    •  i 

which  a  body    distance  which  B,  bodv  will  lall  in  one  second  ot 

falls  be  found?  J 

time. 

Thus,  a  stone  is  five  seconds  in  falling  from  the  top  of  a  precipice ;  the  square 
of  five  seconds  is  25;  this  multiplied  by  16,  the  number,  of  feet  a  body  will 
fall  in  one  second,  gives  400 — the  height  of  the  precipice. 

HOW  do  the  108.  As  the  effect  of  gravity  is  to  produce  a 
SSSamng  uniform  accelerated  motion,  the  velocity  of  a 
compare?  falling  body  will  increase  as  the  time  increases. 

.  *  The  spaces  described  by  falling  bodies  are  here  given  in  round  numbers,  the  fractions 
being  omitted.  The  space  described  by  a  falling  body  during  the  first  second  is  16  l-10th 
feet 

t  The  resistance  of  the  air  essentially  modifies  the  laws  of  the  motions  of  falling  bodies, 
as  here  stated,  and  with  a  certain  velocity,  will  become  equal  to  the  weight  of  the  falling 
body.  After  this  takes  place,  the  body  will  descend  with  a  uniform  velocity.  There 
is,  therefore,  a  limit  to  the  velocity  which  a  body  can  acquire  by  fulling  through  the 
atmosphere. 


56 


WELLS'S   NATURAL   PHILOSOPHY. 


Thus,  at  the  end  of  two  seconds,  the  velocity  acquired  by  a  falling  body 
will  be  twice  as  great  as  at  the  end  of  one  second,  thrice  as  great  at  the  end 
of  the  third  second,  and  so  on. 

109.  Bodies  projected  directly  upward,  will 
i  influenced  by  gravitation  in  their  ascent,  as 
by  gravity?  weii  as  in  ft^  Descent,  but  in  a  reversed 
order ;  producing  continually  retarded  motion  while  they 
are  rising,  and  continually  increasing  motion  during  their 
fall. 

Thus,  a  body  projected  up  perpendicularly  into  the  air,  if  not  influenced  by 
the  resistance  of  the  air,  would  rise  to  a  height  exactly  equal  to  that  from 
which  it  must  have  fallen  to  acquire  a  final  velocity  the  same  as  it  had  at 
the  first  instant  of  its  ascent. 

HOW  can  we  HO.  To  determine  the  height  to  which  a 
heigh™whichba  body  projected  upward  will  rise,  with  a  given 
body  projected  velocity,  ascertain  the  height  from  which  a 


body  would  fall  to  acquire  the  same  velocity. 
The  answer  in  one  case  will  be  the  answer  in 


upardiha 

given  velocity 
will  ascend  ? 

the  other. 

HOW  do  the  HI.  The  time,  also,  which  the  ascending 
andes°desscenf  body  would  require  to  attain  its  greatest 
compare?  height,  would  be  just  equal  to  the  time  it 
would  require  to  fall  to"  the  ground  from  that  height. 

The  following  table  exhibits  an  analysis  of  the  motions  of  a  falling  body; 
the  spaces  passed  over  in  each  interval  of  time  of  falling,  increasing  as  tho 
odd  numbers  1,  3,  5,  7,  9,  etc. ;  the  velocities  acquired  at  the  end  of  each  in- 
terval increasing  directly  as  the  times;  and  the  whole  space  passed  over  being 
as  the  squares  of  the  times. 


Number  of  Seconds 
in  the  Fall,  counted 
from  a  State  of 
Rest. 

Spaces  fallen 
thrWh  in  each 
successive  Second. 

Velochles  acquired 
at.  the  End  of 
Number  of  Seconds 
expressed  in  First 
Column. 

Total  Height  fallen 
through  from  Rest 
in    the   Number  of 
Seconds  expressed  in 
First  Column. 

1 

1 

2 

1 

2 

3 

4 

4 

3 

5 

6 

9 

4 

7 

8 

16 

5 

9 

10 

25 

6 

11 

32 

36 

7 

13 

14 

49 

8 

15 

16 

64 

9 

17 

18 

81 

10 

19 

20 

100 

Where  extreme  accuracy  is  not  required,  most  of  the  problems  connected 
with  the  descent  of  falling  bodies,  may  be  worked  with  great  readiness — 16 


LAWS  OF   FALLING  BODIES.  57 

feet,  the  space  passed  through  by  a  falling  body  in  one  second,  being  taken 
as  the  common  multiple  of  distances  and  velocities. 

Thus,  to  ascertain  the  height  from  which  a  body  would  fall  in  5  seconds, 
take  in  the  fourth  column  of  the  table  the  number  opposite  5  seconds,  which 
is  25,  and  multiply  it  by  16 ;  the  product,  400,  will  be  the  height  required. 
Problems  of  this" character  may  also  be  worked  by  the  rule  given  (§  107). 

In  the  same  manner,  if  it  be  required  to  determine  the  space  a  falling  body 
would  descend  through  in  any  particular  second  of  its  motion,  as,  for  exam- 
ple, the  5th  second,  we  take  in  the  second  column  of  the  table  the  number 
opposite  five  seconds,  which  is  9,  and  multiply  it  by  16  ;  the  product,  144,  is 
the  space  required. 

In  like  manner,  if  it  be  required  to  determine  with  what  velocity  a  body 
would  strike  the  ground  after  falling  during  an  interval  of  5  seconds,  we  take 
the  number  in  the  third  column  of  the  table  opposite  5  seconds,  which  we 
find  to  be  10,  and  multiply  this  by  16.  The  product,  160  feet,  will  be  the 
velocity  required ;  and  a  body  thus  falling  for  5  seconds  would  have,  when 
it  .strikes  the  ground,  a  velocity  of  160  feet. 

112.  If  a  body,  instead  of  falling  perpen- 
the  velocity  of     dicularlv,  be  made  to  roll  down  an  inclined 

a  body  falling  /  '  . 

down  an  in-     plane,  free  from  friction,  the  velocity  acquired 

cliiied  plane?  ,    ,,  -        ,.  t>  •,      j  -iii  i 

at  the  termination  of  its  descent,  will  be  equal 
to  that  it  would  acquire  in  falling  through  the  perpen- 
dicular height  of  the  inclined  plane. 

FIG.  28.  Thus,  the  velocity  acquired  in  rolling  down  the  whole 

length  of  A  B,  Fig.  28,  is  equal  to  that  it  would  acquire 
by  falling  down  the  perpendicular  height  A  C. 

113.  The  great  Italian  philosopher  Galileo,  during  the 
;B  early  part  of  the  17th  century,  had  his  attention  directed, 
while  in  a  church  at  Florence,  to  the  swinging  of  the 

chandeliers  suspended  from  the  lofty  ceiling.     He  noticed  that  when  they 

How,  and   by      were  moved  from  their  natural  position  by  any  disturbing 

whom  was  the      cause,  they  swung  backward  and  forward  in  a  curve,  for  a 

Covered1?1  d         lonS  time»  and  ^th  great  uniformity,  rising  and  felling  alter- 
nately in  opposite  directions.     His  inquiry  into  the  cause  of 

thesD  motions  led  to  the  invention  of  the  pendulum,  the  theory  of  which  may 

be  explained  as  follows : 

Explain  the  114>  ^  bodies  will  have  their  motion  as  much  accelerated 

theory  of  the      whilst  descending  a  curve,  as  retarded  whilst  ascending.    Let 

pendulum.  C  A  B  be  a  curve,  Fig.  29.    If  a 

ball  be  placed  at  C,  the  attraction  of  gravitation 

will  cause  it  to  descend  to  A,  and  in  so  doing  it 

will  acquire  velocity  sufficient  to  carry  it  to  B, 

all  opposing  obstacles  being  removed,  such  as 

friction  and  resistance  of  the  air.    Gravitation 


58 


WELLS'S  NATURAL  PHILOSOPHY. 


How  do  the 
times  of  the  vi- 
brations of  a 
pendulum  com- 
pare with  each 
other? 


will  once  more  bring  it  down  to  A ;  it  will  then  rise  again  to  C,  and  so  con- 
tinue to  oscillate  backward  and  forward. 

If  we  now  suspend  the  ball  by  a  string,  or 
wire,  in  such  a  manner  that  it  will  swing 
freely,  its  motions  will  be  the  same  as  that 
of  the  ball  rolling  upon  the  curve.  A  body 
thus  suspended  is  called  a  PEXDULUM.  In 
Fig.  30,  D  C,  the  part  of  the  circle  through 
which  the  pendulum  moves,  is  called  its  arc, 
and  the  whole  movement  of  the  ball  from  D 
to  C  is  called  an  oscillation. 

115.  The  times  of  the 
vibrations  of  a  pen- 
dulum, are  very  nearly 
equal,     whether      it 

moves    much    or    little ;    or,  in 

other  words,  through  a  greater,  or  less  part  of  its  arc. 

Ex  lain  the  The  reason  that  a  lar&e  vibration  is  performed  in  the  same 

reason  of  this  time  as  a  small  one,  or,  in  other  words,  the  reason  the  pendu- 
law<  lum  always  moves  faster  in  proportion  as  its  journey  is  longer, 

is,  that  in  proportion  as  the  arc  described  is  more  extended,  the  steeper  are 
the  declivities  through  which  it  falls,  and  the  more  its  motion  is  accelerated. 
Thus,  if  a  pendulum,  Fig.  30,  begins  its  motion  at  D,  the  accelerating  force  is 
twice  as  great  as  when  it  is  set  free  at  b  ;  and  if  we  take  two  pendulums  of 
equal  lengths,  and  liberate  one  at  D  and  another  at  b  at  the  same  tune,  they 
will  arrive  at  the  same  moment  at  E. 

116.  This  remarkable  property  of  the  pendulum  enables  us 
to  employ  it  as  a  register,  or  keeper  of  time.    A  pendulum  of 
invariable  length,  and  in  the  same  location,  will  always  make 
the  same  number  of  oscillations  in  the  same  time.     Thus,  if 
we  arrange  it  so  that  it  will  oscillate  once  in  a  second,  sixty 

of  these  oscillations  will  mark  the  lapse  of  a  minute,  and  3,600  an  hour. 

A  common  clock  is,  therefore,  merely  an  arrangement  for 
registering  the   number  of  oscillations  which  a  pendulum 
makes,  and  at  the  same  time  of  communicating  to  the  pendu- 
lum, by  means  of  a  weight,  an  amount  of  motion  sufficient  to  make  up  for 
what  it  is  continually  losing  by  friction  on  its  points  of  support,  and  by  the 
resistance  of  the  air. 

The  wheels  of  the  clock  turn  round  by  the  action  of  the  weight,  but  they 
are  so  connected  with  the  pendulum,  that  with  every  double  oscillation  a  tooth 
of  the  last  wheel  is  allowed  to  pass.  If,  now,  this  wheel  has  thirty  teeth,  as 
is  common  in  clocks,  it  will  turn  round  once  for  every  sixty  vibrations.  And, 
if  the  axis  of  this  wheel  project  through  the  dial-plate  or  face  of  a  clock,  with 
a  hand  fastened  on  it,  this  hand  will  be  the  second  hand  of  the  clock.  The 
other  wheels  are  so  connected  with  the  first,  ^d  the  number  of  teeth  so  pro- 


How  does  this 
property  of  the 
pendulum  en- 
able us  to  reg- 
ister time? 


.  ~    LAWS   OP   FALLING  BODIES.  59 

portioned,  that  -the  second  one  turns  sixty  times  slower  than  the  first,  and 
this  will  be  the  minute  hand  ;  a  third  wheel  moving  twelve  times  slower  than 
the  last  will  constitute  the  hour  hand. 

How  does  a  ^  wa^eu  differs  fr°m  a  clock  in  having  a  vibrating  wheel  in- 
'  watch  differ  stead  of  a  vibrating  pendulum.  This  wheel,  called  the  balance- 
fromaclock?  wheel,  is  moved  by  a  spring,  which  is  always  forcing  it  to  a 
middle  position  of  rest,  but  does  not  fix  it  there,  because  the  velocity  ac- 
yIG  31  quired  during  its  approach  from 

n       .  n  either  side  to  the  middle  position, 

—  carries  it  just  as  far  past  on  the 
other  side,  and  the  spring  has  to 
begin  its  work  again.  The  bal- 
ance-wheel at  each  vibration  allows 


==-  one  tooth  of  the  adjoining  wheel  to 
pass,  as  the  pendulum  does  in  a  clock,  and  the  record  of  the  beats  is  pre- 
served by  the  wheels  which  follow,  as  already  explained  for  the  clock. 

Fig.  3  1  represents  the  arrangement  used  to  keep  up  the  motion  in  a  watch. 
The  barrel,  or  wheel  A,  incloses  a  spring,  which,  when  compressed  by  wind- 
ing up,  tends  to  liberate  itself,  or  unwind,  in  virtue  of  its  elasticity.  This 
effort  to  unwind,  turns  the  barrel  upon  its  axis,  and  thus,  by  means  of  a  chain 
coiled  round  it,  motion  is  communicated  to  the  other  wheels  of  the  watch. 

117.  The  length  of  a  pendulum  influences 

What       influ-  &  .  r 

ence  has  the     the  time  of  its  vibration  :  the  longer  the  pen<- 

length  of  a  pen-  .       '    .,          . 

on  its    dulum  the  slower  are  its  vibrations. 


tion?  °f  Tne  reason  why  long  pendulums  vibrate  more  slowly  than 

short  ones  is,  that  in  corresponding  arcs,  or  paths,  the  ball  of 
the  long  pendulum  has  a  greater  journey  to  perform,  without  having  a  steeper 
line  of  descent. 

What  is  the  118'  ^we  take  a  Pendulum  rod>  Fi£-  32>  ^  A  having  balls 

center  of  oscii-  upon  it  at  C  and  D,  and  cause  it  to  vibrate,  the  ball,  B,  being 
duium?*  P6n"  nearer  to  tne  Point  of  suspension,  will  tend  to  perform 
its  oscillations  more  quickly  than  the  ball  C.  In  like 
manner,  every  other  point  on  the  pendulum  rod  tends  to  complete  its  * 
oscillations  in  a  different  time  ;  but  as  they  are  all  connected  together 
inflexibly,  all  are  compelled  to  perform  their  oscillations  in  the  same 
time.  But  the  action  of  the  portions  of  the  rod  near  to  the  ball,  B, 
is  to  accelerate  the  motion  of  the  pendulum,  and  the  action  of  the 
portions  of  the  rod  near  to  the  ball  C,  is  to  retard  it  ;  therefore  a  point 
may  be  found  where  all  these  counteractions  will  balance  one  an- 
other, or  be  neutralized,  and  this  point  is  termed  the  CENTER  OF  OS- 
CILLATION, and  the  sum  of  the  momenta  of  all  the  portions  of  the  ^  A 
rod  on  each  side  of  this  point  will  balance.  The  center  of  oscillation 
does  not  correspond  with  the  center  of  gravity,  but  is  always  a  little 
below  it  ;  the  practical  method  of  bringing  them  near  together,  is  to 
make  the  rod  light,  and  the  termination  of  the  pendulum  heavy. 


60 


WELLS'S   NATURAL   PHILOSOPHY. 


"Why  do  clocks 
go  faster  in 
winter  than  in 
summer  ? 


How  are  tho 
changes  in  the 
length  of  pen- 
dulums coun- 
'teracted  ? 


FlG.  33. 


FIG.  34 


119.  As  heat  expands,  and  cold  contracts 
all  metals,  a  pendulum  rod  is  longer  in  warm 
than  in  cold  weather  ;  hence,  clocks  gain  time 

in  winter,  and  lose  in  the  summer. 

As  the  smallest  change  in  the  length  of  a 
pendulum  alters  the  rate  of  a  clock,  it  is  highly 
important,  for  the  maintaining  of  uniform  time, 
that  the  expansion  and  contraction  of  pendu- 
lums, caused  by  changes  in  temperature, 
should  be  counteracted.  For  this  purpose  various  contriv- 
ances have  been  employed.  The  one  most  commonly  em- 
ployed at  the  present  time  is  the  mercurial  pendulum,  which, 
is  constructed  as  follows :  The  pendulum  rod,  A  B,  Fig.  33, 
•supports  a  glass  jar,  G  II,  containing  mercury,  inclosed  in  a 
steel  frame- work,  F  C  D  E.  "When  the  weather  is  warm,  the 
steel  rod  and  frame- work  expand,  and  thus  in- 
crease the  length  of  the  pendulum,  and  de- 
press the  center  of  oscillation.  But,  at  the 
same  time,  the  mercury  contained  in  the  jar  also 
expands,  and  rises  upward ;  and  thus,  by  a 
proper  adjustment,  the  center  of  oscillation  is 
carried  as  far  upward  in  one  direction,  as  down- 
ward in  the  opposite  direction,  or  the  expansion 
in  both  directions  is  equal,  and  the  vibrations 
of  the  pendulum  remain  unaltered.  Another  form  of  pendu- 
lum, called  tho  "gridiron  pendulum,"  Fig.  34,  is  composed  of 
rods  of  different  metals,  which  expand  unequally  under  the  same 
changes  of  temperature,  and,  by  counteraction,  keep  the  length 
of  the  pendulum  constant. 

120.  As  the  force  of  gravity  determines  how 
long  the  pendulum  shall  he  in  falling  down  its 
arc,  and  the  time  also  of  its  rising  in  the  op- 
posite direction  (since  the  hall  of  the  pendu- 
lum, as  already  stated,  may  he  considered  as  a  body  de- 
scending hy  its  weight  on  a  slope),  it  follows,  that  the  time 
of  vibration  of  a  pendulum  will  vary  as  the  attraction  of 
gravity  varies. 

The  same  pendulum  will  vibrate  more  slowly  at  the  equa- 
tor than  at  the  poles,  because  the  attraction  of  gravitation  is 
less  powerful  at  the  equator.  Therefore  a  pendulum  to  vi- 
brate once  in  a  second,  must  be  shorter  at  the  equator  than 
at  the  poles.  Corresponding  results  take  place  when  a  pen- 
dulum is  carried  to  a  mountain-top,  away  from  the  center  of  the  earth,  which 


How  do  the 
variations  in 
the  force  of 
gravity  affect 
the  vibrations 
of  a  pendulum? 


Where  will  a 
pendulum  of  a 
given  length 
vibrate  slow- 
est, and  where 
the  fastest  ? 


LAWS  OF  FALLING  BODIES.  :61 

-Is  the  center  of  attraction,  or  when  carried  to  the  bottom  of  a  mine,  where 
it  is  attracted  both  by  matter  above  it  and  below  it. 

what  is  the  121.  The  length  of  a  pendulum  that  will 
Iend*h0pendu-  describe  sixty  oscillations  in  a  minute,  each 
ium?  oscillation  having  the  duration  of  a  second, 

is,  in  the  latitude  of  Greenwich,  England,  39.1393  inches 
.in  length  ;  one  to  vibrate  in  half  seconds  must  measure 
9.7848,  or  rather  more  than  9£  inches. 

At  the  pole  it  would  require  to  be  somewhat  longer ;  at  the  equator  some- 
what shorter.  A  pendulum  that  vibrated  seconds  at  Paris,  was  found  to  re- 
quire lengthening  .09  of  an  inch  in  order  to  perform  its  vibrations  in  the  same 
time  at  Spitzbergen. 

.HOW  may  the  122.  The  length  of  a  pendulum  vibrating 
seconds  being  always  invariable  at  the  same 
P^ce,  since  the  attraction  under  the  same 

measure?          circumstances  is  always  the  same,  it  may  be 

used  as  a  standard  of  measure. 

This  application  has  already  been  described  under  the  section  Weight  (§  67). 

The  duration  of  the  oscillation  of  a  pendulum  is  not  affected  by  altering  the 
weight  of  the  ball,  since  all  bodies  moving  over  the  same  space,  under  the 
influence  of  gravitation,  acquire  equal  velocities. 

HOW  do  the  123.  The  lengths  of  different  pendulums, 
deuium s  v\ br&t.  vibrating  in  unequal  times,  are  to  each  other 
SolJmpa6^?  as  tiie  squares  of  the  times  of  their  vibration. 

Thus  a  pendulum,  to  vibrate  once  in  two  seconds,  must 
have  four  times  the  length  of  one  that  vibrates  once  in  one  second ;  to  vibrate 
once  in  three  seconds,  it  must  have  nine  times  the  length,  etc. — the  duration 
of  the  oscillation  being  as  the  whole  numbers, 

1,  2,  3,  4,  5,  6,  7,  8,  9. 
"The  length  of  the  pendulum  will  be  as  their  squares. 

1,  4,  9,  16,  25,  36,  49.  64,  81. 

A  pendulum,  therefore,  that  will  vibrate  once  in  nine  seconds,  must  have 
'a  length  of  81  times  greater  than  one  vibrating  once  in  one  second. 

PRACTICAL  PROBLEMS  ON  THE  THEORY  OF  FALLING 
BODIES. 

1.  A  stone  let  fall  from  the  top  of  a  tower  struck  the  earth  in  two  seconds;  how  high 
was  the  tower  ? 

2.  How  far  will  a  body  acted  upon  by  gravity  alone,  fall  in  ten  seconds  ? 

3.  How  deep  is  a  well,  into  which  a  stone  being  dropped,  reaches  the  surface  of  the 
water  in  two  seconds,  the  depth  of  the  water  in  the  well  being  ten  feet? 


62  WELLS'S  NATURAL  PHILOSOPHY. 

,    4.-  If  a  body  be  projected  downward  with  a  Telocity  of  twenty- two  feet  in  the  first  sec- 
ond of  time,  how  far  will  it  fall  in  eight  seconds  ? 
The  multiple  in  this  case  will  be  the  distance  fallen  through  in  the  first  second. 

5.  What  space  will  a  body  pass  through  in  the  fourth  second  of  its  time  of  falling  ? 

6.  A  body  falls  to  the  ground  in  eight  seconds ;  how  large  a  space  did  it  pass  over  dur- 
ing the  last  second  of  its  descent  ? 

7.  A  body  falls  from  a  height  in  eight  seconds  ;  with  what  velocity  did  it  strike  the 
ground  ? 

8.  A  cannon-ball  fired  upward,  continued  to  rise  for  nine  seconds;  what  was  its  velocity 
during  the  first  second,  or  with  what  force  was  it  projected  ? 

9.  Suppose  a  bullet  fired  upward  from  a  gun  returned  to  the  earth  in  sixteen  seconds ; 
how  high  did  it  ascend  ? 

The  time  occupied  in  ascending  and  descending  being  equal,  the  body  rose  to  such  a 
height  that  it  required  eight  seconds  to  descend  from  it.  The  square  of  8=64.  This 
multiplied  by  the  space  it  would  fall  in  the  first  second,  16  feet  =  924  feet. 

10.  A  bird  wag  shot  while  flying  in  the  air,  and  fell  to  the  ground  in  three  seconds. 
How  high  up  was  the  bird  when  it  was  shot  ? 

11.  What  must  be  the  length  of  a  pendulum  to  vibrate  once  in  seven  seconds  ? 

12.  If  the  length  of  a  pendulum  to  vibrate  seconds  at  Washington  is  39.101  inches,  how 
long  must  it  be  to  vibrate  half  seconds  ?    How  long  to  vibrate  quarter  seconds  ? 


CHAPTER   7. 

MOTION. 

what  is  MO-        124.  MOTION  is  the  act  of  changing  place. 

tion  ?  If  no  motion  existed,  the  universe  would  be  dead.     There 

would  be  no  alternation  of  the  seasons,  and  of  day  and  night ;  no  flow  of 
water,  or  change  of  air ;  no  sound,  light,  heat,  or  animal  existence. 

125.   MOTION  is  ABSOLUTE  or  RELATIVE. 
solute  aid  Rei-    ABSOLUTE  MOTION  is  a  change  of  position  in 

ative  Motion?  .,          ,          .,,  ~i  A 

space,  considered  without  reference  to  any 
other  body.  RELATIVE  MOTION  is  motion  considered  in 
relation  to  some  other  body,  which  is  either  in  motion  or 
at  rest. 

Thus  the  motions  of  the  planets  in  space  are  examples  of  Absolute  Motion, 
but  the  motion  of  a  man  sitting  upon  the  deck  of  a  vessel,  while  sailing,  is 
an  example  of  Relative  Motion,  since  he  is  in  motion  as  respects  the  land, 
but  at  rest  as  regards  the  parts  of  the  vessel.  Rest,  which  is  the  opposite 
of  motion,  so  far  as  we  know,  exists  only  relatively.  "We  say  a  body  on  the 
surface  of  the  earth  is  at  rest,  when  it  maintains  a  constant  position  as  re- 
gards some  other  body ;  but  at  the  same  time  that  it  is  thus  at  rest,  it  partakes 


MOTION.  63 

of  the  motion  of  the  earth,;  -which  is  always  revolving.  "We  do  not,  therefore, 
really  know  any  body  to  be  in  a  state  of  absolute  rest. 

Define  Uni-  126.  A  moving  body  may  have  a  UNIFORM 
Sable  Motilr  or  a  VARIABLE  MOTION.  UNIFORM  MOTION  is 
the  motion  of  a  body  moving  over  equal 
spaces  in  equal  times.  VARIABLE  MOTION  is  the  mo- 
tion of  a  body  moving  over  unequal  spaces  in  equal 
times. 

.what  is  AC-  127.  When  the  spaces  passed  over  in  equal 
Retted  M£  times  increase,  the  body  is  said  to  possess  Ac- 
tion?  CELERATED  MOTION  ;  when  they  diminish,  the 

body  is  said  to  possess  RETARDED  MOTION. 

A  stone  falling  through  the  air  is  an  example  of  Accelerated  Motion,  since, 
acted  upon  by  the  force  of  gravity,  its  rate  of  motion  constantly  increases  ; 
while  the  ascent  of  a  stone  projected  from  the  hand,  is  an  example  of  Re- 
tarded Motion,  its  upward  motion  continually  decreasing. 

what  is  Power  128.  When  a  body  commences  to  move  from 
aSce?  Resi8t"  a  state  of  rest,  we  assign  some  force  as  the 
cause  of  its  motion  ;  and  a  force  acting  in  such 
a  manner  as  to  produce  motion,  is  generally  termed 
"  POWER."  On  the  contrary,  a  force  acting  in  such  a  way 
as  to  retard  a  moving  body,  destroy  its  motion,  or  drive 
it  in  a  contrary  direction,  is  termed  RESISTANCE.  The 
chief  forces  which  tend  to  retard  or  destroy  the  motion  of 
a  body  are  GRAVITATION.  FRICTION,  and  RESISTANCE  OF 
THE  AIR. 

what  is  ve-        129.  The  speed,  or  rate,  at  which  a  body 
locity?        moves,  is  termed  its  VELOCITY. 

Moving  bodies  pass  over  their  paths  with  different  degrees  of  speed  ;  one 
may  pass  through  ten  feet  in  a  second  of  time,  and  another  through  a  hun- 
dred feet  in  the  same  period.  We  say,  therefore,  that  they  have  different 
velocities. 

The  velocity  of  a  moving  body  is  estimated  by  the  time  it  occupies  in 
moving  over  a  given  space,  or  by  the  space  passed  over  in  a  given  time.  Tho 
less  the  time  and  the  greater  the  space  moved  over  in  that  time,  the  greater 
the  velocity. 

now  do  we        130.  To  ascertain  the  VELOCITY  of  a  mov- 
body,  divide  the  space  passed  over  by  the 


tb 


veioctty  Ofa  , 

moving  body?    ftmQ  consume(i  jn  moving  over  it. 


64  WELLS'S  NATURAL  PHILOSOPHY. 

Thus,  if  a  body  moves  10  miles  in  2  hours,  its  velocity  is  found  by  di- 
viding the  space,  10,  by  the  time,  2  ;  the  answer,  5,  gives  the  velocity  per 
hour. 

131.  To  ascertain  the  SPACE  passed  over  by 
a  moving  body,  multiply  the  velocity  by  the 

over  by  a  body      f^p 
in  motion?  lime. 

Thus,  if  the  velocity  be  10  miles  per  hour,  and  the  time  15 
hours,  the  space  will  be  10  multiplied  by  15,  or  150  miles. 

HOW  is  the  132.  To  ascertain  the  TIME  employed  by  a 

b™ea  biS^lJ    body  in  motion,  divide  the  space  passed  over 

potion   ascer-       ty  tne  velocity. 

Thus,  if  the  space  passed  over  be  150  miles,  and  the  ve- 
locity 10  miles  per  hour,  the  whole  time  employed  will  be  150  divided  by 
10  =  15  hours. 

what  is  MO-        133.  The  MOMENTUM  of  a  body  is  its  quan- 
mentum?        fjty  Of  motjon.      Momentum    expresses   the 
force  with  which  one  body  in  motion  would  strike  against 
another. 

That  a  mass  of  matter  moving  in  any  manner  exerts  a  cer- 
^am  ^orce  against  any  object  with  which  it  may  come  in  con- 
tact, is  a  principle  of  Natural  Philosophy  which  experience 
teaches  us  most  frequently  and  most  readily.  The  child  has  hardly  emerged 
from  the  nurse's  arms,  before  it  becomes  conscious  of  the  force  with  which 
it  would  strike  the  ground  if  it  fell.  We  take  advantage  of  momentum,  or 
the  force  of  a  moving  body,  in  almost  all  mechanical  operations.  The  mov- 
ing mass  of  a  hammer-head  drives  or  forces  in  the  nail,  shapes  the  iron,  breaks 
the  stone  ;  the  force  of  a  moving  mass  of  water  gives  strength  to  a  torrent. 
and  turns  the  wheel  ;  the  force  of  a  moving  mass  of  air  gives  strength  to  the 
wind,  carries  the  ship  over  the  ocean,  forces  round  the  arms  of  a  wind-mill. 

is  motion  im-  134.  When  a  body  is  caused  to  move,  the 
niotion  is  not  imparted  simultaneously  to 
evei7  particle  of  the  body,  but  at  first  only  to 

stant?  the  particles  which  are  directly  exposed  to  the 

influence  of  the  force  —  for  instance,  of  a  blow.     From 

these  particles,  it  spreads  to  the  rest. 

A  slight  blow  is  sufficient  to  smash  a  whole  pane  of  glass, 
How  can  you 
illustrate  this     while  a  bullet  from  a  gun  will  only  make  a  small  round  hole 

foct?  in  it,  because,  in  the  latter  case,  the  particles  of  glass  that  re- 

.ceive  the  blow  are  torn  away  from  the  remainder  with  such  rapidity,  that  the 
motion  imparted  to  them  has  no  time  to  spread  further.  A  door  standing  open, 
which  would  readily  yield  on  its  hinges  to  a  gentle  push,  is  not  moved  by  a 
cannon-ball  passing  through  it.  The  ball,  in  passing  through,  overcomes  the 


theasamtyin- 


MOTION.  65 

whole  force  of  cohesion  among  the  atoms  of  wood,  but  its  force  acts  for  so 
short  a  time,  owing  to  its  rapid  passage,  that  it  is  not  sufficient  to  affect  the 
inertia  of  the  door  to  an  extent  to  produce  motion.  The  cohesion  of  the  part 
of  the  wood  cut  out  by  the  ball  would  have  borne  a  very  great  weight  laid 
quietly  upon  it;  but  supposing  the  ball  to  fly  at  the  rate  of  1200  feet  in  a 
second,  and  the  door  to  be  one  inch  thick,  the  cohesion  being  allowed  to  act 
for  only  the  minute  fraction  of  a  second,  its  influence  is  not  perceived. 
k  It  is  an  effect  of  this  same  principle,  that  the  iron  head  of  a  hammer  may  be 
driven  down  on  its  wooden  handle,  by  striking  the  opposite  end  of  the 
handle  against  any  hard  substance  with  force  and  speed.  In  this  very  simple 
operation,  the  motion  is  propagated  so  suddenly  through  the  wood  of  the  han- 
dle, that  it  is  over  before  it  can  reach  the  iron  head,  which  therefore,  by  its 
own  inertia,  sinks  lower  on  the  handle  at  every  blow,  which  drives  the  han- 
dle up. 

HOW  is  the  MO-  135.  The  MOMENTUM,  or  force,  which  a  mov- 
ing  body  exerts,  is  estimated  by  multiplying 
j£S  mass  or  quantity  of  matter  by  its  velocity. 

Thus,  a  body  weighing  10  pounds,  and  moving  with  a  velocity  of  500  feet 
in  a  second,  will  have  a  momentum  of  (10X500)  5,000. 

what  connec  ^^'  ^e  ve^oc^y  being  the  same,  the  mo- 
tion is  there  mcntum,  or  moving  force  of  a  body,  will  he 

between      the          .  .  J ' 

Momentum  _of    directly  proportionate  to  the  mass,  or  weight  ; 
weight  and  ve-    and  the  mass  or  weight  remaining  the  same, 
the  momentum  will  be  directly  proportionate 
to  the  velocity. 

Thus,  if  2  leaden  balls,  each  of  5  pounds'  weight,  move  with  a  velocity  of 
5  miles  per  minute,  the  momentum,  or  striking  force  of  each,  will  be  25 ; 
if  now  the  two  balls,  molded  into  one  of  10  pounds'  weight,  move  with  the 
same  velocity  of  5  miles  per  minute,  the  momentum,  or  striking  force,  will 
be  50,  since  with  the  same  velocity  the  mass,  or  weight,  will  be  doubled.  If, 
on  the  contrary,  we  double  the  velocity,  allowing  the  weight  to  remain  the 
same,  tlie  same  effect  will  be  produced ;  a  ball  of  5  pounds,  with  a  velocity 
of  5,  will  have  a  momentum,  or  striking  force,  of  25  ;  but  a  ball  of  5,  with  a 
velocity  of  10,  will  have  a  momentum  of  50. 

HOW  can  a  137.  A  small,  or  light  body,  may  be  made 
nStionbb°edraade  to  strike  with  a  greater  force  than  a  heavier 
sameXforcelsa  body,  by  giving  to  the  small  body  a  sufficient 

large  one?  velocity. 

Illustrations  of  these  principles  are  most  familiar.  Hail-stones,  of  small 
mass  and  great  velocity,  strike  with  sufficient  force  to  break  glass,  and  de- 
stroy standing  grain ;  a  ship  of  huge  mass,  moving  with  'a  scarcely  percept- 
ible velocity,  crushes  in  the  side  of  the  pier  with  which  it  comes  in  contact. 


66  WELLS'S  NATURAL  PHILOSOPHY. 


SECTION    I. 

ACTION      AND      REACTION. 

138.  When  a  body  communicates  motion 

What  is  meant  »  . 

by  Action  and    to  another  body,  it  loses  as  much  of  its  own 

Reaction?  J '  .  .  . 

momentum,  or  force,  as  it  gives  to  the  other 
body.  We  apply  the  term  ACTION  to  designate  the  power 
which  a  body  in  motion  has  to  impart  motion,  or  force,  to 
another  body  ;  and  the  term  REACTION  to  express  the 
power  which  the  body  acted  upon  has  of  depriving  the 
acting  body  of  its  force,  or  motion. 

what  is  the        139.  There  is  no  motion,  or  action,  in  the 
and  Re-     universe  without  a  corresponding  and  oppo- 

gj£e  action  of  equal  amount  ;  or,  in  other  words, 
ACTION  and  REACTION  are  always  equal  and  opposed  to 
each  other. 

What  »re  H-  ^  a  Person  Presses  the  table  'with  his  finger,  he  feels  a  re- 
lustrations  of  sistance  arising  from  the  reaction  of  the  table,  and  this  coun- 
acti<m  ?*nd  Re"  ter-pressure  is  equal  and  contrary  to  the  downward  pressure. 
When  a  cannon  or  gun  is  fired,  the  explosion  of  the  powder 
which  gives  a  forward  motion  to  the  ball,  gives  at  the  same  time  a  backward 
motion,  or  "  recoil,"  to  the  gun.  A  man  in  rowing  a  boat,  drives  the  water 
astern  with  the  same  force  that  he  impels  the  boat  forward. 

TO  what  is  the        1^0.  The  quantity  of  motion  in  a  body  is 
f  a    measured  by  the  velocity  and  the  quantity  of 
?ropor~    matter  it  contains. 

A  cannon-ball  of  a  thousand  ounces,  moving  one  foot  per 
second,  has  the  same  quantity  of  motion  in  it  as  a  musket-ball  of  one  ounce, 
leaving  the  gun  with  a  velocity  of  a  thousand  feet  per  second.  The  momen- 
tum, or  quantity  of  motion,  in  the  musket-ball  being,  however,  concentrated 
in  a  very  small  mass,  the  effect  it  will  produce  will  be  apparently  much 
greater  than  that  of  the  cannon-ball,  whose  motion  is  diffused  through  a  very 
large  mass.  This  explanation  will  enable  us  to  understand  some  phenomena 
which  at  first  appear  to  contradict  the  law,  that  action  and  reaction  are  always 
equal,  and  opposed  to  each  other. 

Thus,  when  we  fire  a  bullet  from  a  gun,  the  gun  recoils  back  with  as  much 
force  as  the  bullet  possesses,  proceeding  in  an  opposite  direction.  The  reason 
the  effects  of  the  gun  are  not  equally  apparent  with  those  of  the  ball,  is  that 
the  motion  of  the  gun  is  diffused  through  a  great  mass  of  matter,  with  a 
small  velocity,  and  is,  therefore,  easily  checked ;  but  in  the  ball  the  motion 


ACTION   AND    REACTION. 


67 


is  concentrated  in  a  very  small  compass,  with  a  great  velocity.  A  gun  recoils 
more  with  a  charge  of  fine  shot,  or  sand,  than  with  a  bullet.  The  explanation 
of  this  is,  that  with  a  ball  the  velocity  is  communicated  to  the  whole  mass 
at  once,  but  with  small  shot,  or  sand,  the  velocity  communicated  by  the  ex- 
plosion to  those  particles  of  the  substance  immediately  in  contact  with  the  powder, 
is  greater  than  that  received  at  the  same  instant  by  the  outer  particles;  con- 
sequently, a  larger  proportion  of  explosive  force  acts  momentarily  in  an  oppo- 
site  direction. 

FIG.  35. 


"We  have  an  illustration  of  this  same  principle,  when  we  attempt  to  drive  a 
nail  into  a  board  having  no  support  behind  it,  or  not  sufficiently  thick  to  offer 
the  necessary  resistance  to  the  moving  force  of  the  hammer,  as  is  repre- 
sented in  Fig.  35.  The  blows  of  the  hammer  will  cause  the  board  to  unduly 
yield,  and  if  strong  enough,  will  break  it,  but  will  not  drive  in  the  nail.  The 
object  is  attained  by  applying  behind  the  board,  as  in  Fig.  36,  a  block  of  wood, 

FIG.  36. 


68  WELLS'S  NATURAL  PHILOSOPHY. 

or  metal,  against  which  the  blows  of  the  hammer  will  be  directed.  By 
adopting  this  plan,  however,  no  increased  resistance  is  opposed  to  the  blows 
of  the  hammer,  the  momentum,  or  moving  force  of  which  is  equally  imparted 
in  both  cases ;  but  in  the  first  case,  the  momentum  is  received  by  the  board 
alone,  which,  having  little  weight,  is  driven  by  it  through  so  great  a  space 
as  to  produce  considerable  flexure,  or  even  fracture;  but  in  the  second  case, 
the  same  momentum  being  shared  between  the  board  and  the  block  behind  it, 
will  produce  a  flexure  of  the  board  as  much  less  as  the  weight  of  the  board 
and  block  applied  to  it  together,  is' greater  than  the  weight  of  the  board  alone. 
The  same  principle  serves  to  explain  a  trick  sometimes  exhibited  in 
feats  of  strength,  where  a  man  in  a  horizontal  position,  his  legs  and 
shoulders  being  supported,  sustains  a  heavy  anvil  upon  his  chest,  which 
is  then  struck  by  sledge-hammers.  The  reason  the  exhibitor  sustains  no 
injury  from  the  blows,  is  that  the  momentum  of  the  sledge  is  distributed 
equally  through  the  great  mass  of  the  anvil,  and  gives  to  the  anvil  a  down- 
ward motion,  just  as  much  less  than  the  motion  of  the  sledge,  as  the  mass  of 
the  sledge  is  less  than  the  mass  of  the  anvil.  Thus,  if  the  weight  of  the  an- 
vil be  100  times  greater  than  the  weight  of  the  sledge,  its  downward  motion 
upon  the  body  of  the  exhibitor  will  be  100  times  less  than  the  motion  with 
which  the  sledge  strikes  it,  and  the  body  of  the  exhibitor  easily  yielding  to 
so  slight  a  movement,  and  also  resisting  it  by  means  of  the  elasticity  of  the 
body,  derived  from  its  peculiar  position,  escapes  without  injury. 

when  is  the  141.  When  two  bodies  come  in  contact,  the 
Todii°nBafd1to  collision  is  said  to  be  direct,  when  a  right  line 
be  direct?  passing  through  their  centers  of  gravity  passes 
also  through  the  point  of  contact. 

•  The  center  of  gravity  in  such  cases  corresponds  with  the  center  of  col- 
lision ;  and  if  such  a  center  come  against  an  obstacle,  the  whole  momentum 
of  the  body  acts  there,  and  is  destroyed ;  but  if  any  other  part  hit,  the  body 
only  loses  a  portion  of  its  momentum,  and  revolves  round  the  obstacle  as  a 
pivot,  or  center  of  motion. 

when  two  in-        142.  When  two  non-elastic  bodies,  moving 
comecinto°coci!    m  opposite  directions,  come  into  direct  collision, 
cul-s1? whatoc"    they  will  each  lose  an  equal  amount  of  mo- 
mentum. 

Hence,  the  momentum  of  both  after  contact,  will  be  equal  to  the  difference 
of  the  momenta  of  the  two  before  contact,  and  the  velocity  after  contact  will 
be  equal  to  the  difference  of  the  momenta  divided  by  the  whole  quantity  of 
matter.  Let  the  quantity  of  matter  in  A  be  2,  and  its  velocity  12  ;  its  mo- 
mentum is,  therefore,  24.  Let  the  quantity  of  matter  in  B  be  4,  its  velocity 
3;  its  momentum  will  be  12.  The  momentum  of  the  mass  after  contact,  on 
the  supposition  they  move  in  opposite  directions  and  come  in  direct  col- 
lision, will  be  the  difference  of  the  two  momenta,  or  12 ;  and  the  velocity  of 


ACTION    AND   KEACTION. 


69 


Explain  the  re- 
sults of  the  col- 


FIG.  37. 


To  what  will 
the  shock  of 
collision  of  two 
bodies  coming 
in  contact  be 
equivalent  ? 


the  mass  will  be  its  momentum  divided  by  the  quantity  of  matter,  or  1 2  di- 
vided by  6,  which  is  2.* 

If  two  non-elastic  bodies,  as  A  and  B,  Fig.  37,  be  suspended  from  a  fixed 
point,  and  the  one  be  raised  toward  Y,  and  the  other  toward  X,  an  equal 
amount,  they  will  acquire  an  equal  force,  or  momentum,  in  falling  down  the 

arc,  provided  their  masses  are  equal; 

and  will  by  contact  destroy  each 

other's    motion,  and  come  to  rest. 

If  their  momenta  are  unequal,  they 
will,  after  contact  move  on  together,  in  the  direction 
of  the  body  having  the  largest  quantity  of  motion 
with  a  momentum  equal  to  the  difference  of  the 
momenta  of  the  two  before  collision. 

143.  The  force  of  the 
shock    produced    by    two 
equal     bodies    coming    in 
contact  with  equal  velocity, 

will  be  equal  to  the  force  which  either,  being  at  rest,  would 
sustain,  if  struck  by  the  other  moving  with  double  the 
velocity  ;  for  reaction  and  action  being  equal,  each  of  the 
two  will  sustain  as  much  shock  from  reaction  as  from  ac- 
tion. 

If  a  person  running,  come  in  contact  with  another  who  is 
standing,  both  receive  a  certain  shock.  If  both  be  running 
at  the  same  rate  in  opposite  directions,  the  shock  is  doubled. 

In  combats  of  pugilists,  the  most  severe  blows  are 

those  struck  by  fist  against  fist,  for  the  force  sustained 

by  each  in  such  cases,  is  equal  to  the  sum  of  the 

forces  exerted  by  the  two  arms.  If  two  ships,  mov- 
ing in  contrary  directions  at  the  rate  of  20  miles  per 

hour,  come  in  collision,  the  shock  will  be  the  same  as 

if  one  of  them,  being  at  rest,  were  struck  by  the 

other,  moving  at  40  miles  per  hour. 

144.  If  we  suspend  two  balls  of 
some  non-elastic  substance,  as  clay  or 
putty,  by  strings,  so  that  they  can 
move  freely,   and   allow  one  of  the 

balls  to  fall  upon  the  other  at  rest,  it  will  communicate  to 
it  a  part  of  its  motion,  and  both  balls,  after  collision,  will  move  on  together. 


FIG.  38. 


If  one  inelastic 
body  comes  in 
contact  with 
another  at  rest, 
what  occurs  ? 


•  This  whole  subject,  usually  considered  dry  and  uninteresting,  will  be  found  to  possess 
a  new  interest,  if  the  student  will  make  himself  a  few  simple  experiments,  by  suspending 
leaden  balls  by  the  side  of  a  graduated  arc,  as  in  Fig.  37,  and  allow  them  to  fall  under 
different  conditions.  The  length  of  the  arc  through  which  they  fall  will  be  found  to  bo 
an  exact  measure  of  the  force  with  which  they  will  strike. 


70 


WELLS'S   NATURAL   PHILOSOPHY. 


The  quantity  of  motion  will  remain  unchanged,  the  one  having  gained  as 
much  as  the  other  has  lost ;  so  that  the  two,  if  equal,  will  have  half  the  ve- 
locity after  collision  that  the  moving  one  had  when  alone.  Fig.  38  represents 
two  balls  of  clay,  E  and  D,  non-elastic,  of  equal- weight,  suspended  by  strings. 
If  the  ball  D  be  raised  and  let  fall  against  the  ball  E,  a  part  of  its  motion  will 
be  communicated  to  E,  and  both  together  will  move  on  to  e  d. 
When  two  145-  lf  we  suspend  two  balls,  A  and  B,  Fig.  39,  of  some 

elastic  bodies      elastic  substance,  as  ivory,  and  allow  them  to  fall  with  equal 

come  into  col-  _         ,   , 

listen,  what  oc-     masses  and  velocities  from  the  points  X  and  Y  on  the  arc, 

they  will  not  come  to  rest  after  collision,  but  will  recede 


FiG.  39. 


What    occa- 
sions the  dif- 
ference in  the 
results  of  the 
collision     of 
elastic  and  non. 
elastic  bodies? 


curs? 

from  each  other  with  the  same  velocity  which  each 
had  before  contact. 

The  reason  of  this  movement  in 
highly  elastic  bodies,  contrary  to 
what  takes  place  in  non-elastic 
bodies,  is  this:  the  elastic  sub- 
stances are  compressed  by  the  force 
of  the  shock,  but  instantly  recover- 
ing their  former  shape  in  virtue  of  their  elasticity, 
they  spring  back,  as  it  were,  and  react,  each  giving 
to  the  other  an  impulse  equal  to  the  force  which 
caused  its  compression. 

Suppose  the  ball  A,  however,  to  strike  upon  the 
ball  B  at  rest ;  then,  after  impact,  A  will  remain  at 

rest,  but  B  will  move  on  with  the  same  volocity  as  A  had  at  the  moment  of 
contact.  In  this  case  the  reaction  of  elasticity  causes  the  ball  A  to  stop,  and 
the  ball  B  to  move  forward  with  the  motion  which  A  had  at  the  instant  of 
contact. 

The  same  fact  may  be  illustrated 
by  suspending  a  number  of  elastic 
balls  of  equal  weight,  as  represented 
in  Fig.  40.  If  the  ball  H  be  drawn 
out  a  certain  distance,  and  let  fall 
upon  G,  the  next  in  order,  it  will 
communicate  its  motion  to  G,  and 


OOOCOOO 


FIG.  40. 


receive  a  reaction  from  it,  which  will 
destroy  its  own  motion.  But  the 
bal1  B  can  not  move  without  moving 
F;  it  will,  therefore,  communicate 
the  motion  it  received  from  G  to  F,  and  receive  from  F  a  reaction  which  will 
stop  its  motion.  In  like  manner,  the  motion  and  reaction  are  received  by  each 
of  the  balls  E,  D,  C,  B,  A,  until  the  last  ball,  K,  is  reached  ;  but  there  being 
no  ball  beyond  K  to  act  upon  it,  K  will  fly  off  as  far  from  A,  as  H  was 
drawn  apart  from  G. 


REFLECTED   MOTION. 


71 


SECTION    II. 


REFLECTED     MOTION. 


mat  is  Re- 

fleeted  Motion? 


in  what  man. 


be  reflected? 


What  is  the 
Angle  of  Inci- 
dence? 


What  is  the 
Angle  of  Re- 
flection ? 


146.  When  any  elastic  body,  as  an  ivory 
11,  is  thrown  against  a  hard  smooth  surface, 
the  reaction  will  cause  it  to  rebound  from  such  surface, 
and  the  motion  it  receives  is  called  REFLECTED  MOTION. 
14T.  If  the  ball  be  projected  perpendicu- 
it  will  rebound  in  the  same  direction  ; 
be  projected  obliquely,  it  will  rebound 
obliquely  in  an  opposite  direction,  making  the  angle  of 
incidence  equal  to  the  angle  of  reflection. 

148.  The  ANGLE  of  INCIDENCE  is  the  angle 
formed  by  the  line  of  incidence  with  a  perpen- 
dicular to  any  given  surface. 

149.  The    ANGLE    of  REFLECTION  is   the 
angle  formed  by  the  line  of  reflection  with  a 
perpendicular  to  any  given  surface. 

41.  Thus,  in  Fig.  41,  let  B  E  be  a  smooth,  flat 

surface.  If  the  ball,  A,  be  projected,  or  thrown 
upon  this  surface,  in  the  direction  A  C,  it  will 
rebound,  or  be  reflected,  in  the  direction  C  F. 
In  this  case,  the  line  A  C  is  the  line  of  inci- 
dence, and  the  angle  A  C  D,  which  it  makes 
with  a  perpendicular  D  C,  is  the  angle  of  inci- 
C  ~  denca  In  like  manner  the  line  C  F  is  the  line 

of  reflection,  and  the  angle  D  C  F  the  angle  of 

reflection.  If  the  ball  be  projected  against  the  surface,  B  C,  in  the  direction 
D  C,  perpendicular  to  the  surface,  it  will  be  reflected,  or  rebound  back  in  the 
same  straight  line.  .  .; 

150.  The  ANGLES  of  INCIDENCE  and  RE- 
FLECTION are  always  equal  to  one  another. 

Thus,  in  Fig.  41,  the  angles  A  C  D  and  F  C  D  are  equal. 

151.  An  ANGLE  is  simply  the  inclination  of 
the  lines  which  meet  each  other  in  a  point. 
The  size  of  the  angle  depends  upon  the  open- 
ing, or  inclination,  of  the  lines,  and  not  upon  their  length. 


What  propor- 
tion exists  be- 
tween the  an- 
gles of  incidence 
and  reflection  ? 

What  is  an 
Angle,  and  up- 
on what  does 
its  size  depend  ? 


72 


"WELLS'S   NATURAL   PHILOSOPHY. 


silts*  the  Sdii  The  skill  of  tho  player  of  billiards  and  bagatelle  depends 
of  the  Game  of  upon  his  dexterous  application  of  the  principles  of  incident 
Billiards?  an(j  reflected  motion,  which  he  ha«  learned  by  long-continued 

experience,  viz.,  that  the  angle  of  incidence  is  always  equal  to  the  angle 
of  reflection,  and  that  action  and  reaction  are  equal  and  contrary.  An  illus- 
tration of  the  skillful  reflection  of  billiard  balls  is  given  in  Fig.  42,  which  rep- 
resents the  top  of  a  billiard-table.  The  ball,  P,  when  struck  by  the  stick,  Q, 

FiG.  42. 


is  first  directed  in  the  line  P  0,  upon  the  ball  P',  in  such  a  manner,  that  being 
reflected  from  it,  it  strikes  the  four  sides  of  the  table  successively,  at  the  points 
marked  0,  and  is  finally  reflected  so  as  to  strike  the  third  ball,  P".  At  each 
of  the  reflections  from  the  ball  P',  and  the  four  points  on  the  side  of  the  table, 
the  angle  of  incidence  is  exactly  equal  to  the  angle  of  reflection. 

152.  Imperfectly  elastic  bodies  oppose  the 
momentum  of  bodies  in  motion  more  perfectly 
than  any  others,  in  consequence  of  their  yield- 
ing to  the  force  of  collision  without  reacting  ; 
opposing  a  gradual  resistance  instead  of  a  sud- 
den one. 

Hence  a  feather-bed,  or  a  sack  of  wool,  will  stop  a  bullet  much  more  ef- 
fectually than  a  plate  of  iron,  from  its  deadening,  as  it  is  popularly  called,  the 
force  of  the  blow. 


Eare   im- 
ctly  elas- 
idies  pecu- 
liarly fitted  to 
oppose  and  de- 
stroy momen- 
tum? 


SECTION    III. 

COMPOUND    MOTION. 


What  is  Sim- 
ple Motion  ? 


153.  A  body  acted  upon  by  a  single  force, 
moves  in  a  straight  line,  and  in  the  direction 
of  that  force.  Such  motion  is  designated  as  SIMPLE  MO- 
TION. 


COMPOUND    MOTION.  73 

A  body  floating  upon  the  water  is  driven  exactly  south  by 
pie  Motion™"      a  wind  blowing  south.     A  ball  fired  from  a  cannon  takes  the 
exact  direction  of  the  bore  of  the  cannon,  or  of  the  force 
which  impels  it. 

what  is  com-  1^4.  When  a  body  is  acted  upon  by  two 
pound  Motion?  forces  at  the  same  time,  and  in  different  di- 
rections, as  it  can  not  move  two  ways  at  once,  it  takes  a 
middle  course  between  the  two.  Such  motion  is  termed 
COMPOUND  MOTION. 

155.  The   course  in  which  a  body,  acted 

What    is    the  .          .       *' 

course   of   a    upon  by  two  or  more  forces,  acting  in  different 

body  acted  up-  £          .  J  .  „     ..      .      °-~ 

on    by    two    directions,  will  move,  is  called  the  KESULTANT, 

forces  called?  .        _     J    ,    .          -~ .  '        . 

or  the  Resulting  Direction. 

In  Fig.  43,  if  a  "body,  A,  be  acted  upon 
at  the  same  time  by  two  forces,  one  of 
which  would  cause  it  to  move  in  the  di- 
rection A  Y,  over  the  space  A  B,  in  one 
second  of  time,  and  the  other  cause  it  to 
move  in  the  direction  A  X,  over  the  space 
A  C,  in  one  second ;  then  the  two  forces, 
acting  upon  it  at  the  same  instant,  will 
cause  it  to  move  in  a  Resultant  Direction, 
A  D,  in  one  second.  This  direction  is  the 
diagonal  of  a  parallelogram,  which  has  for  its  sides  the  lines  A  B,  A  0,  over 
which  the  body  would  move  if  acted  upon  by  each  of  the  forces  separately. 

156.  The  operations  of  every-day  life  afford  numerous  exam- 
iSua-      pies  of  Resultant  Motion.     If  we  attempt  to  row  a  boat  across 

^  Result-  a  Ta^  river,  the  boat  will  be  subjected  to  action  of  two  forces  ; 
viz.,  the  action  of  the  oars,  which  tend  to  drive  it  across  the 
river  in  a  certain  time,  as  ten  minutes,  in  a  straight  line,  as  from  A  to  B,  Fig. 
43,  and  the  action  of  the  current,  which  tends  to  carry  it  down  the  stream  a 
certain  distance  in  the  same  time,  as  from  A  to  C.  It  will,  therefore,  under 
the  influence  of  both  these  forces,  move  diagonally  across  the  river,  or  in  the 
direction  A  D,  and  arrive  at  D  at  the  expiration  of  the  ten  minutes.  When 
we  throw  a  body  from  the  deck  of  a  boat  in  motion,  or  from  a  railroad  car. 
the  body  partakes  of  the  motion  of  the  boat  or  the  car,  and  does  not  strike  at 
the  point  intended,  but  is  carried  some  distance  beyond  it.  For  the  same  rea- 
son, in  firing  a  rifle  from  the  deck  of  a  vessel  moving  rapidly,  at  some  object 
at  rest  upon  the  bank,  allowance  must  be  made  for  the  motion  of  the  vessel, 
and  aim  directed  behind  the  object. 

157.  The  principles  of  the  composition  and 

What    is    the  ,      '.  £    ,./«  „ 

science  of  Pro-    resolution  of  different  forces  acting  upon  a 
body  to  produce  motion,  constitute  the  basis 


74 


WELLS'S   NATURAL   PHILOSOPHY. 


What  is  a  Pro- 


body     thro 
obliquely  ? 


PIG.  44. 


of  the  SCIENCE  of  PROJECTILES,  or  that  department  of 
Natural  Philosophy  which  considers  the  motion  of  bodies, 
thrown  or  driven  by  an  impelling  force  above  the  surface 
of  the  earth. 

1^8.  A  PROJECTILE  is  a  body  thrown  into 
ajr  in  any  direction  ;  as  a  stone  from  the 
laand,  or  a  ball  from  a  gun,  or  cannon. 

What  is  the  di  If  we  Pr°Ject  a  bodv  perpendicularly  downward,  or  upward, 
rection  of  a  it  will  move  in  a  perpendicular  line  with  a  uniform  accelerated 
or  retarded  motion,  since  the  force  of  gravity  and  that  of  pro- 
jection are  in  the  same  line  of  direction.  But  if  a  body  is 
thrown  in  a  direction  oblique  to  the  perpendicular,  it  is  acted  upon  by  two 
forces,*  the  projectile  force  which  tends  to  impel  it  forward  in  a  straight  line, 
and  the  force  of  gravity,  which  tends  to  bring  it  to  the  earth.  Instead,  there- 
fore, of  following  the  direction  of  the  projectile  force,  the  path  of  the  body 
will  be  a  curve,  the  resultant  of  the  two  forces.  Such  a  curve  is  called  a 
PARABOLA. 

If  a  cannon-ball  is  fired  from  A  to- 
ward B,  Fig.  44,  in  an  upward  direction, 
instead  of  moving  along  the  line  A  B, 
it  is,  by  the  influence  of  the  earth's  at- 
traction, continually  drawn  downward, 
and  its  path  is  along  a  line  which  is  in- 
dicated by  the  parabolic  curve  A  C; 
and  although  it  has  been  moving  on- 
ward from  the  impulse  it  has  received 
from  the  force  of  the  gunpowder,  it  oc- 
cupies exactly  the  same  time  in  falling 
to  the  point  C,  as  if  the  ball  had  been  allowed  to  drop  from  the  hand  at  A, 
and  fall  to  D. 

159.  If  a  ball  be  projected  from  the  mouth 
of  a  cannon  in  a  horizontal  direction,  it  will 
reach  the  earth  in  precisely  the  same  time  as 
a  ball  dropped  from  the  mouth  of  the  gun.  The  force 
of  gravity  is  neither  increased  or  diminished  by  the  force 
of  projection. 

The  same  fact  may  be  strikingly  illustrated  by  placing  a  number  of  marblea 
at  unequal  distances  from  the  edge  of  a  table  and  sweeping  them  off  with  a 
ruler,  or  stick  :  those  which  are  rolled  along  the  farthest  will  be  projected  the 
farthest;  yet  all  will  strike  the  floor  at  the  same  time. 

*  The  theoretical  laws  governing  the  motion  of  projectiles,  as  herewith  given,  are  In 
practice  essentially  modified  by  the  resistance  of  the  air. 


What  effect  has 
the  projectile 
force  on  the 
action  of  grav- 
ity? 


COMPOUND   MOTION. 


75 


FIG.  45. 


FIG.  46. 


Suppose  from  the  point  A,  Fig.  45,  about  240  feet 
above  the  earth,  a  ball  to  be  projected  in  a  perfectly 
horizontal  line,  A  B  ;  instead  of  traversing  this  line, 
it  would,  at  the  end  of  the  first  second,  be  found 
that  the  ball  had  fallen  15  feet,  at  the  same  time  it 
had  moved  onward  in  the  direction  of  B.  Its  true 
position  would  be,  therefore,  at  a ;  at  the  end  of  tho 
second  second,  it  would  have  passed  onward,  but 
have  fallen  to  &,  60  feet  below  the  horizontal  line  • 
and  at  the  end  of  the  third  second,  it  would  have 
fallen  135  feet  below  the  line,  and  be  at  c;  and  thus 
it  would  move  forward  and  reach  the  earth  at  d  240  feet,  in  precisely  the  same 
tune  it  would  "have  occupied  in  falling  from  A  to  C. 

An  oblique,  or  horizontal  jet  of  water,  is  an 
instance  of  the  curve  described  by  a  body  act- 
ed upon  by  gravity  and  the  force  of  projection. 
See  Fig.  46. 

160.  The  KANGE  of 
a     projectile,    is    the 
horizontal  distance  to 

which  it  can  be  thrown. 

161.  According    to 
theory,   the    range    is 
greatest  when  the  angle 

of  elevation  is  45° ;  and  is  the  same  for  elevations  equally 
above  and  below  45° ;  as  for  example  70°  and  20°.  See 
Fig.  47. 

These  conclusions  are,  however,  found  to 
be  essentially  modified  in  practice  by  the 
resistance  of  the  air,  which  not  only  changes 
the  path  but  the  velocity  of  the  projectile. 
With  great  velocities,  as  in  the  case  of  a 
cannon-ball,  the  greatest  range  corres- 
ponds with  an  elevation  of  about  30°,  but 
for  slow  motions  it  is  near  45°. 

162.  The  laws  of 
projectiles    are    es- 
pecially regarded  in 
the  art  of  gunnery. 

By  knowing  the  force  of  the  powder  which  drives  the  ball, 
the  engineer  is  enabled  to  direct  the  cannon,  or  mortar, 
in  such  a  manner  as  to  cause  the  ball5  or  bomb,  to  fall 


What  is  the 
Range  of  a 
Projectile? 


How  can  the 
greatest  Range 
be  obtained  ? 


FlG.  47. 


How  are  the 
Laws  of  Pro- 
jectiles practi- 
cally applied 
in  military  en- 
gineering? 


76 


WELLS'S   NATURAL   PHILOSOPHY. 


upon  a  particular  spot  in  the  distance  ;  thus  producing  a 
desired  effect  without  a  wasteful  expenditure  of  ammuni- 
tion. 

FIG.  48. 


Fig.  48  represents  a  bombardment,  and  the  three  lines  indicate  the  curves 
made  by  the  balls.  If  the  bombardment  had  been  conducted  from  an  eleva- 
tion, instead  of  the  level  surface,  the  balls  would  have  gone  beyond  the  city, 
as  shown  by  the  familiar  fact,  that  we  can  throw  a  heavy  body  to  a  greater 
distance  from  an  elevation,  as  the  steep  bank  of  a  river,  than  on  a  plain,  or 
level  ground.  It  was  on  this  principle  that  Napoleon  bombarded  Cadiz,  at  the 
distance  of  five  miles,  and  from  a  greater  elevation,  the  balls  could  have  been 
thrown  to  a  still  greater  distance.* 

*  The  following  facts  respecting  the  explosive  force  of  gunpowder,  and  its  application  to 
projectiles,  will  be  found  interesting  and  instructive  in  this  connection.  The  estimated 
force  of  gunpowder  when  exploded,  is  at  least  14,750  pounds  upon  every  square  inch  of 
the  surface  which  confines  it.  Count  Rumford  showed,  by  his  experiments  made  about 
60  years  ago,  that  if  the  powder  were  placed  in  a  close  cavity,  and  the  cavity  two  thirds 
filled,  its  dimensions  being  at  the  same  time  restricted,  the  force  of  explosion  would  ex- 
ceed 150,000  pounds  upon  the  square  inch. 

The  force  of  gunpowder  depends  upon  the  fact,  that  when  brought  in  contact  with  any 
ignited  substance,  it  explodes  with  great  violence.  A  vast  quantity  of  gas,  or  elastic  fluid, 
is  emitted,  the  sudden  production  of  which,  at  a  high  temperature,  is  the  cause  of  the 
riolent  effects  which  are  produced. 

The  reason  that  gunpowder  is  manufactured  in  little  grains,  is  that  it  may  explode  more 
quickly,  by  facilitating  the  passage  of  the  flame  among  the  particles.  In  the  form  of  dust, 
the  particles  would  be  too  compact. 

The  velocity  of  balls  impelled  by  gunpowder  from  a  musket  with  a  common  charge,  ha» 
been  estimated  at  about  1,650  feet  in  a  second  of  time,  when  first  discharged.  The  utmost 
Velocity  that  can  be  given  to  a  cannon-ball  is  2,000  feet  per  second,  and  this  only  at  the' 
moment  of  its  leaving  the  gun.  ,-v»  1 

In  order  to  increase  the  velocity  from  1,650  to  2,000  feet,  one  half  more  powder  is  re- 
quired; and  even  then,  at  a  long  shot,  no  advantage  is  gained,  since,  at  the  distance  of 
600  yards,  the  greatest  velocity  that  can  be  obtained  is  only  1,200  or  1,300  feet  per  second. 
Great  charges  of  powder  are,  therefore,  not  only  useless,  but  dangerous ;  for,  though  they 
give  little  additional  force  to  the  ball,  they  hazard  the  lives  of  many  by  their  liability  to 
burst  the  gun.  The  velocity  is  greater  with  long  than  with  short  guns,  because  the  influ- 
ence of  the  powder  upon  the  ball  is  longer  continued. 

The  essential  properties  of  a  gun  are  to  confine  the  elastic  fiuid  generated  by  the  explo- 
sion of  the  powder  as  completely  as  possible,  and  to  direct  the  course  of  the  ball  in  a 


COMPOUND   MOTION.  77 

According  to  the  laws  which  govern  the  motion  of  project- 
gmTbeh  a?med  iles>  it;  is  evident  tllat  a  gun  must  be  aimed,  in  order  to  hit 
to  hit  an  ob-  an  object,  in  a  direction  above  that  of  the  object,  more  or  less, 
distance*?8™*1  according  to  the  distance  of  the  object  and  the  force  of  the 
charge.  With  an  aim  directed,  as  in  Fig.  49,  at  the  object, 
the  ball,  moving  in  a  curved  path,  must  necessarily  fall  below  it. 

straight,  or  rectilinear  path.  A  rifle  sends  a  ball  more  accurately  than  a  musket,  becanse 
the  ball  is  in  more  accurate  contact  with  the  sides  of  the  barrel  than  in  the  case  of  a  com- 
mon  musket  The  space  produced  by  the  difference  of  diameter  between  the  ball  and  the 
lore  of  the  gun,  greatly  diminishes  the  effect  of  the  powder,  by  allowing  a  part  of  the 
elastic  fluid  to  escape  before  the  ball,  and  also  permits  the  ball  to  deviate  from  a  straight 
line.  The  peculiarity  and  superiority  of  the  new  rifle,  called  the  "  Minic  rifle,"  is  to  be 
found  in  the  construction  of  the  ball,  which,  by  the  act  of  firing,  is  made  to  fit  completely 
the  barrel,  or  bore,  of  the  gun.  This  is  accomplished  by  making  the  ball  of  an  oblong 
shape  and  a  conical  point,  with  an  opening  in  the  base  extending  up  for  two  thirds  the 
length  of  the  ball.  Into  the  opening  of  this  internal  cylinder  there  is  placed  a  small  con- 
cave section  of  iron,  which  the  powder,  at  the  moment  of  firing,  forces  into  the  leaden 
ball  with  great  power,  spreading  it  open,  and  causing  it  to  fit  tightly  to  the  cavity  of  the 
barrel  in  its  course  out,  thus  giving  it  a  perfect  direction. 

Cannon  of  different  sizes  are  named  according  to  the  weight  of  the  ball  which  they  are 
capable  of  discharging.  Thus,  we  have  68-pounders,  24-pounders,  18-pounders,  and  the 
lighter  field-pieces,  from  4  to  12-pounders.  The  quantity  of  powder  generally  used  for 
discharging  common^ron  or  brass  cannon,  is  one  third  the  weight  of  the  ball.  In  gen- 
eral warfare,  the  effective  distance  at  which  artillery  can  be  used  is  from  500  to  600  yards, 
or  from  a  quarter  to  half  a  mile.  At  the  battle  of  Waterloo,  the  brigades  of  artillery  were 
stationed  about  half  a  mile  from  each  other.  Cannon-balls  and  shells  can  be  thrown 
with  effect  to  the  distance  of  a  mile  and  a  half  to  two  miles. 

The  distance  to  which  a  ball  may  be  thrown  by  a  24-pounder,  with  a  quantity  of  powder 
equal  to  two  thirds  the  weight  of  the  ball,  is  about  four  miles.  Its  effective  range  is,  how- 
ever, much  less.  Were  the  resistance  of  the  air  entirely  removed,  the  same  ball  would 
be  thrown  to  about  five  times  that  distance,  or  twenty  miles. 

It  has  been  found  that,  by  the  firing  of  an  18-pound  shot  into  a  butt,  or  target,  made  of 
beams  of  oak,  when  the  charges  were  6  pounds  of  powder,  3  pounds,  2$  pounds,  and  1 
pound,  the  respective  depths  of  the  penetration  were  42  inches,  30  inches,  28  inches,  and 
15  inches;  and  the  velocities  at  which  the  balls  flew,  were  1,600  feet  in  a  second,  1,140 
feet,  1,024  feet,  and  &>Qfeet. 

•  When  the  cannon  is  so  pointed  that  the  ball  goes  perfectly  straight  toward  the  object 
aimed  at,  the  direction  is  said  to  be  point-blank.     Ricochet  firing  is  when  the  ball  is  dis- 
charged in  such  a  manner  that  it  goes  bounding  and  skipping  along  the  surface  of  the 
ground.    In  this  way  a  ball  can  be  thrown  more  effectively,  and  for  a  greater  distance, 
than  in  any  other  way. 

There  are  several  substances  known  to  chemists  which  possess  a  greater  explosive 
power  than  gun  powder.  It  has  not,  however,  been  considered"  possible  to  increase  the 
range  and  effect  of  a  projectile  fired  from  a  gun,  or  cannon,  by  using  any  of  them.  Sup- 
posing that  the  guns  could  be  made  indefinitely  strong,  and  the  gunpowder  indefinitely 
powerful,  the  point  would  soon  be  reached  where  the  resistance  which  the  air  opposes  to 

•  body  moving  very  rapidly  would  balance  the  force  derived  from  the  explosive  compound, 
which  drives  the  projectile  forward.     Beyond  this  point  no  increase  of  impulsive  force 
would  urge  the  projectile  farther ;  and  this  limit  is  considerably  within  the  range  of 
power  that  can  be  exercised  by  common  gunpowder.    Beside  this,  the  strength  of  mate- 
rials of  which  guns  are  made  is  limited.     Practical  experience  has  fully  demonstrated  that 
the  largest  piece  of  ordnance  which  can  be  cast  perfect,  sound,  and  free  from  flaws,  is  a 
mortar  13  inches  diameter;  and  even  this  weighs  five  tons.    The  French,  at  the  siege  of 
Antwerp,  constructed  a  mortar  having  a  bore  of  no  less  than  20  inches  diameter,  but  it 
burst  on  the  ninth  time  of  firing. 


78 


WELLS'S   NATURAL   PHILOSOPHY. 

FIG.  49. 


• ...a.... 


Until  quite  recently,  the  muskets  placed  in  the  hands  of  soldiers  were  usu- 
ally aimed  so  that  the  line  of  sight  was  parallel  to  the  barrel,  and  directed  to 
the  object,  as  in  Fig  49.  So  long  as  the  range  of  the  musket  was  of  limited 
extent,  and  great  precision  was  not  expected,  the  deviation  of  the  ball  from 
a  straight  line  was  not  taken  into  account ;  but  with  the  introduction  of  rifles 
throwing  a  ball  to  a  great  distance,  the  drop  of  the  ball  occasioned  by  the 
curvature  of  the  path  of  the  projectile,  was  found  to  deprive  the  weapon  of 
,the  necessary  precision.  On  all  modern  guns,  therefore,  a  double  sight  is 
.  provided,  by  which  the  elevation  necessary  to  secure  accurate  aim  can  always 
be  given  to  the  barrel.  This  is  exhibited  in  Fig.  50,  where  one  of  the  sights, 
B,  is  fixed,  in  the  usual  manner,  at  one  extremity  of  the  barrel,  while  the 
other  is  located  nearer  the  breach.  This  last  sight  is  often  graduated  and 
provided  with  an  adjustment,  by  which  it  can  be  adapted,  to  objects  at  dif- 
ferent distances,  so  as  to  hit  them  exactly. 

FIG.  50. 


What  is  cir-  163.  CIRCULAR  MOTION  is  the  motion  pro- 
cuiar  Motion?  &UCQ&  by  the  revolution  of  a  body  about  a 
central  point. 

164.  Circular  Motion  is  a  species  of  com- 

How  IB  Clrcu-  .  .  -i-it  • 

lar  Motion  pro-  pound  motion,  and  is  caused  by  the  continued 
operation  of  two  forces  ; — one  the  force  of 
projection,  which  gives  the  body  motion,  tends  to  cause 
it  to  move  in  a  straight  line  ;  while  the  other  is  continually 
deflecting  it  from  a  straight  course  toward  a  fixed  point. 

IlluBtrat     th  ^kk  ^ac*  ^S  illustrated  by  the  common  sling,  or  by  swinging 

production   of     a  heavy  body  attached  to  a  string  round  the  head.     The  body, 
tior»CUlar    M°~      m  this  case'  moves  through  the  influence  of  two  forces,  the 
force  of  projection,  and  the  string  which  confines  it  to  the 
hand.    These  two  forces  act  at  right  angles  to  one  another,  and  according  to 


COMPOUND    MOTION.  79 

tho  statements  already  made  (§  155),  the  path  of  the  moving  body  will  be  a 
resultant  of  the  two  forces,  or  the  diagonal  of  a  parallelogram. 

How  then,  it  may  be  asked,  does  the  body  attached  to  the 
curve  of  a  cir-  string  and  whirled  round  the  head,  move  in  a  circle  ?  This 
credos  equivl-  wili  k6  clear>  if  we  consider  that  a  circle  is  made  of  an  in- 
lent  to  the  finite  number  of  little  straight  lines  (diagonals  of  parallelo- 
paralldo"rama?  grams)  and  that  the  body  moving  in  it,  has  its  motion  bent 
at  every  step  of  its  progress  by  the  action  of  the  force  which 
confines  it  to  the  hand  This  force,  however,  only  keeps  it  within  a  certain 
distance,  without  drawing  it  nearer  to  the  hand.  The  two  forces  exactly 
balancing  each  other,  the  course  of  the  whirling  body  will  be  circular. 

what  are  the         165.  The  two  forces  by  which  circular  mo- 
is  produced,  are  called  the  CENTRIFUGAL* 

CENTRIPETAL  ForCCS.f 

166.  The  CENTRIFUGAL  FORCE  is  that  force 
which  impels  a  body  moving  in  a  curve  to 
move  outward,  or  fly  off  from  a  center. 

16T.  The  CENTRIPETAL  FORCE  is  that  force 

What  if?  Cen-  .  •  .'       •  .  , 

tripetai Force?  which  draws  a  body  moving  in  a  curve  toward 
the  center,  and  assists  it  to  move  in  a  bent,  or  curvelinear 
course.  In  Circular  Motion  the  Centrifugal  and  Centri- 
petal Forces  are  equal,  and  constantly  balance  each  other. 

If  the  Centrifugal  Force  of  a  body  revolving  in  a  circular 
W1the  CentTi-  Path  be  destr°7ed>  tne  bod7  wil1  immediately  approach  the 
fugal  or  Cen-  center ;  but  if  the  Centripetal  Force  be  destroyed,  the  body 
-are destroyed?  wil1  flv  off  in  a  straight  line,  called  a  tangent 

Thus,  in  whirling  a  ball  attached  by  a  string  to  the  fin- 
ger, the  propelling  force,  or  the  force  of  projection,  is  given  by  the  hand,  and 
the  Centripetal  Force  is  exhibited  in  the  stretching,  or 
tension  of  the  string.  If  the  string  breaks  in  whirling, 
the  Centripetal  Force  no  longer  acts,  and  the  ball,  by 
the  action  of  the  Centrifugal  Force,  generated  by  the 
whirling  motion,  flies  off  in  a  tangent,  or  straight  line, 
as  is  represented  in  Fig.  51.  If,  on  the  contrary,  the 
whirling  motion  is  too  slow,  the  Centripetal  Force  pre- 
ponderates, and  the  ball  falls  in  toward  the  finger. 

Familiar  examples  of  the  effects  of  Centrifugal  Force 
are  common  in  the  experience  of  every-day  life. 

Whit  are  fa  ^e  mo^on  °^  mud  %mn  fr°m  t^e  r'im  °^  a  cc-ach- wheel, 

miiiar  iiiustra-      moving  rapidly,  is  an  illustration  of  Centrifugal  Force.     Fig. 
1"      52  rePresents  a  coach- wheel  throwing  off  mud ;  a  the  point  at 
which  the  mud  flies  off;  a  &,  the  straight  line  in  which  it 
*  Centrifugal,  compounded  of  center,  and  "/»«/£o,"  to  fly  off. 
t  Centripetal,  compounded  of  center  and  "peto"  to  seek. 


80 


WELLS'S   NATURAL   PHILOSOPHY. 


•would  move  but  for  the  action  of  the  two  forces,  which  compel  it  to  follow 
the  parabolic  curve,  a  c. 

FIG.  52. 

I 


Tinder  what 
circumstances 
will  the  Cen- 
trifugal Force 
overcome  the 
Force  of  Cohe- 
sion? 


FlG.  53. 


The  mud  sticks  to  the  wheel,  in  the  first  instance,  through  the  force  of  ad- 
hesion; .but  this  force,  being  very  weak,  is  overcome  by  the  Centrifugal  Force, 
and  the  particles  of  mud  fly  off.  The  particles  which  compose  the  wheel  it- 
self would  also  fly  off  in  the  same  mamner,  were  not  the  force  of  cohesion 
which  holds  them  together  stronger  than  the  Centrifugal  Force. 

The  Centrifugal  Force,  however,  increases  with  the  velocity 
of  revolution,  so  that  if  the  velocity  of  the  wheel  were  contin- 
ually increased,  a  point  would  at  last  be  reached,  when  the 
Centrifugal  Force  would  be  more  powerful  than  the  force  of 
cohesion,  and  the  wheel  would  then  fly  in  pieces.     In  this 
way  almost  all  bodies  can  be  broken  by  a  sufficient  rotative 
velocity.     Large  wheels  and  grind- 
stones, revolving  rapidly,  not  infre- 
quently break  from  this  cause,  and  the 
pieces  fly  off  with  immense  force  and 
velocity. 

When  we  whirl  a  mop,  the  water 
flies  off  from  it  by  the  action  of  the 
Centrifugal  Force.  The  fibers,  or 
threads,  which  compose  the  mop,  also 
tend  to  fly  off,  but  being  confined  at 
one  end,  they  are  unable  so  to  do. 
They,  therefore,  assume  a  spherical 
form,  or  shape. 

The  fact  that  water  can  be  made  to 
fly  off  from  a  mop,  by  the  action  of  the 
Centrifugal  Force  produced  by  whirling 
it,  has  been  most  ingeniously  applied 
in  a  machine  for  drying  cloth,  called 


COMPOUND   MOTION. 


81 


FIG.  54. 


the  hydro-extractor  (water-extractor),  Fig.  53.  The  machine  consists  of  a 
large  hollow  wheel,  or  cylinder,  A  A,  turning  upon  an  axis,  B.  The  sides 
and  bottom  of  the  wheel  are  pierced  with  holes  like  a 
sieve.  The  wet  cloths  being  in  and  around  the  sides, 
A,  the  wheel  is  caused  to  revolve  with  great  rapidity,  and 
the  water  contained  in  the  material,  by  the  action  of  the 
Centrifugal  Force,  flies  out,  and  escapes  through  the 
apertures  left  in  the  sides  of  the  wheel.  A  rotation 
of  1500  times  per  minute,  is  sufficient  to  almost  en- 
tirely dry  the  cloth,  no  matter  how  wet  it  may  have  been 
originally. 

When  a  bucket  of  water,  attached  to  a  string,  is 
whirled  rapidly  round,  the  water  does  not  fall  out  when 
the  mouth  is  presented  downward,  since  the  Centrifugal 
Force  imparted  to  the  water  by  rotation,  tends  to  cause 
it  to  fly  off  from  the  center,  and  this  overcomes,  or  bal- 
ances, the  attraction  of  gravitation,  which  tends  to  cause 
the  water  to  fall  out,  or  toward  the  center.  Thus,  in 
Fig  54,  the  water  contained  in  the  bucket  which  is  up- 
side down,  has  no  support  under  it,  and  if  the  bucket 
were  kept  still  in  its  inverted  position  for  a  single  mo- 
ment the  water  would  fall  out  by  its  own  weight,  or,  in 
other  words,  by  the  attraction  of  gravitation,  which  rep- 
resents a  Centripetal  Force ;  but  the  Centrifugal  Force, 
which  is  caused  by  the  whirling  of  the  bucket  in  the  di- 
rection of  the  arrow,  tends  to  drive  the  water  out  through 
the  bottom  and  side  of  the  vessel,  and  as  this  last  force 
overcomes,  or  balances  the  other,  the  water  retains  its 
place,  and  not  a  drop  is  spilled. 

"When  a  carriage  is  moved  rapidly  round  a  corner,  it  is 
very  liable  to  be  overturned  by  the  Centrifugal  Force 
called  into  action.  The  inertia  carries  the  body  of  the 
vehicle  forward  in  the  same  line  of  direction,  while  the 
wheels  are  suddenly  pulled  around  by  the  horses  into  a 
new  one.  Thus  a  loaded  stage  running  south,  and  sud- 
denly turned  to  the  east,  throws  out  the  luggage  and 
passengers  on  the  south  side  of  the  road.  "When  railways  form  a  rapid  curve, 
the  outer  rail  is  laid  higher  than  the  inner,  in  order  to  counteract  the  Centri- 
fugal Force. 

An  animal,  or  man,  turning  a  corner  rapidly,  leans  in  toward  the  corner  or 
center  of  the  curve  in  which  ho  is  moving,  in  order  to  resist  the  action  of 
the  Centrifugal  Force,  which  tends  to  throw  him  away  from  the  center. 

In  ah1  equestrian  feats  exhibited  in  the  circus,  it  will  be  observed  that  not 
only  the  horse,  but  the  rider,  inclines  his  body  toward  the  center,  Fig.  55,  and 
according  as  the  speed  of  the  horse  round  the  ring  is  increased,  this  inclina- 
'tibn  becomes  more  considerable.  "When  the  horse  walks  slowly  round  a  large 


82 


WELLS'S   NATURAL  PHILOSOPHY. 


ring  this  inclination  of  his  body  is  imperceptible;  if  he  trot,  there  is  a  visible 
inclination  inward,  and  if  he  gallop,  ho  inclines  still  more,  and  when  urged  to 
full  speed  he  leans  very  far  over  on  his  side,  and  his  feet  will  be  heard  to 
strike  against  the  partition  which  defines  the  ring.  The  explanation  of  all 
this  is,  that  the  Centrifugal  Force  caused  by  the  rapid  motion  around  the 
ring  tends  to  throw  the  horse  out  of,  and  away  from,  the  circular  course,  and 
this  he  counteracts  by  leaning  inward. 


FIG.  65. 


How  do  the 
motions  of  the 
solar  system  il- 
lustrate the  ac- 
tion of  Centri- 
fugal and  Cen- 
tripetal Forces  ? 


The  most  magnificent  exhibition  of  Centrifugal  and  Centri- 
petal Forces  balancing  each  other,  is  to  be  found  in  the  ar- 
rangements of  the  solar  system.  The  earth  and  other  planets 
are  moving  around  a  center — the  sun,  with  immense  veloci- 
ties, and  are  constantly  tending  to  rush  off  into  space,  by  the 
action  of  the  Centrifugal  Force.  They  are,  however,  restrained 

within  exactly  determined  limits  by  the  attraction  of  the  sun,  which  acts 

as  a  centripetal  power  drawing  them  toward  the  center. 

what  is  the        168.  The  Axis  of  a  body  is  the  straight  line, 
of  a  body?   reaj  Qr  imaginarv  ?  passing  through  it,  on  which 
it  revolves,  or  may  revolve. 

169.  When  a  body  rotates  upon  an  axis,  all 
its  parts  revolveJn  equal  times.  The  velocity 
of  each  particle  of  a  revolving  body  increases 
with  its  perpendicular  distance  from  the  axis, 
and  as  its  velocity  increases,  its  Centrifugal  Force  in- 
creases. 

A  moment's  reflection  will  show,  that  a  point  on  the  outer  .part,  or  rim,  of 
a  wheel,  moves  round  the  axis  in  the  same  time  as  a  point  nearer  the  center, 
as  upon  the  hub.  But  the  circle  described  by  the  revolution  of  the  outer  part 


When  a  hody 
revolves  round 
its  Axis,  what 
peculiarities 
do  its  several 
parts  exhibit  ? 


COMPOUND    MOTION. 


83 


effect 
does  the  action 
of  Centrifugal 


a  body? 


FlG.  56. 


of  the  wheel  is  much  larger  than  that  described  by  the  inner  part,  and  as 
both  move  round  the  center  in  the  same  time,  the  outer  part  must  move  with 
a  greater  velocity. 

170.  If  the  particles  of  a  rotating  body  have 
freedom  of  motion  among  themselves,  a  change 

in  the  figure  of  the  bod7  may  be  occasioned  by 
ti^  difference  Of  the  Centrifugal  Force  in  the 
different  parts. 

A  ball  of  soft  clay,  with  a  wire  for  an  axis,  forced  through  its  center,  if  made 

to  turn  quickly,  soon  ceases  to  be  a  perfect  ball.     It  bulges  out  in  the  middle, 

where  the  Centrifugal  Force  is,  and  becomes  flattened  toward  the  ends,  or 

where  the  wire  issues. 

This  change  in  the  form  of 
revolving  bodies  may  be  illus- 
trated by  an  apparatus  repre- 
sented in  Fig.  56.  This  con- 
sists of  an  elastic  circle,  or  hoop, 
fastened  at  the  lower  side  on  a 
vertical  shaft,  while  the  upper 
side  is  free  to  move.  On  turn- 
ing the  wheels,  so  arranged  as 
to  impart  a  very  rapid  motion 
to  the  shaft  and  hoop,  the  hoop 
will  be  observed  to  bulge  out 
in  the  middle  (owing  to  the 
Centrifugal  Force  acting  with 

greater  intensity  upon  those  parts  furthest  removed  from  the  axis)  and  to  be- 

come flattened  at  the  ends. 

What   is  th  ^^*  ^ie  ear^1  itself  is  an  example  of  the  operation  of  this 

cause  of  the  force.  Its  diameter  at  the  equator  is  about  twenty-six  miles 
greater  tlian  its  Polar  diameter.  The  earth  is  supposed  to 
have  assumed  this  form  at  the  commencement  of  its  revolu- 

tion, through  the  action  of  the  Centrifugal  Force,  while  its  particles  were  in  a 

semi-fluid,  or  plastic  state.     In  Fig.  57  we  -pIG  ^ 

have  a  representation  of  the  general  figure  of 

the  earth,  in  which  N  S  is  the  polar  diameter, 

and  also  the  axis  of  rotation,  and  E  W  the 

equatorial  diameter. 

172.   At  the  equator  the 
What    is    the 
Amount  of  Cen-     Centrifugal  Force  of  a  particle 

°f  mattdtr>  is   l-290ths  of  its 

gravity.     This  diminishes  as 
we  approach  the  poles,  where  it  becomes  0. 
If  the  earth  revolved  17.  times  faster  than 
it  now  does,  or  in  84  minutes  instead  of  24 


84  WELLS'S  NATURAL  PHILOSOPHY. 

What  would  be     hours,  the  Centrifugal  Force  would  be  equal  to  the  attraction 

the  effect  if  the        „      .     .     x. 

Telocity  of  ro-     °J  gravitation,  which  may  be  considered  as  the  Centripetal 

earSTwere  in°     Force'  and  a11  bodies  at  the  earth's  equator  would  be  deprived 
creased  ?  of  weight,  since  they  would  have  as  great  a  tendency  to  leave 

the  surface  of  the  earth  as  to  descend  toward  its  center.  If 
the  earth  revolved  on  its  axis  in  less  time  than  84  minutes,  terrestrial  gravita- 
tion would  be  completely  overpowered,  and  all  fluids  and  loose  substances 
would  fly  from  its  surface. 

173.  There  appears  to  be  a  constant  tendency  to  rotary 
motion  in  moving  bodies  free  to  turn  upon  their  axes. 
The  earth  turns  upon  its  axis,  as  it  moves  in  its  orbit  ;  a 
ball  projected  from  a  cannon,  a  rounded  stone  thrown  from 
the  hand,  all  revolve  around  their  axes  as  they  move. 

FlG.  58.  This  phenomenon  may  be  very 

prettily  illustrated  by  placing  a 
watch-glass  upon  a  smooth  plate 
of  glass,  Fig.  58,  moistened  suf- 
ficiently to  insure  slight  adhesion, 
and  fixed  at  any  angle.  As  it 
begins  to  move  toward  the  bot- 
\jm  of  the  inclined  plane,  it  will  exhibit  a  revolving  motion,  which  uniformly 
increases  with  the  acceleration  of  its  downward  movement. 


PRACTICAL    QUESTIONS   AND   PROBLEMS   ON  THE  PRINCIPLES 
AND   COMPOSITION   OF   MOTION. 

1.  The  BUBFAOE  of  the  EARTH  at  the  EQUATOE  mores  at  the  rate  of  about  a  THOUSAND 
MILES  in  an  HOUR  :  why  are  MEN  not  sensible  of  this  rapid  movement  of  the  earth  ? 

Because  all  objects  about  the  observer  are  moving  in  common  with  him.  It 
is  the  natural  uniformity  of  the  undisturbed  motion  which  causes  the  earth 
and  ah1  the  bodies  moving  together  with  it  upon  its  surface  to  appear  at 
rest. 

2.  How  can  you  easily  see  that  the  EABTH  is  in  motion  ? 

By  looking  at  some  object  that  is  entirely  unconnected  with  it,  as  the  sun 
or  the  stars.  We  are  here,  however,  liable  to  the  mistake  that  the  sun  or 
Btars  are  in  motion,  and  not  wo  ourselves  with  the  earth. 

3.  Does  the.  SUN  really  BISE  and  SET  each  day  ? 

The  sun  maintains  very  nearly  a  constant  position ;  but  the  earth  revolves, 
and  is  constantly  changing  its  position.  Really,  therefore,  tJie  sun  neither  rises 
nor  sets. 

4.  Why,  to  a  PEBSON  SAILING  in  a  BOAT  on  a  smooth  stream,  or  GOING  SWIFTLY  in  a 
CABBIAGE  on  a  smooth  road,  do  the  trees  or  buildings  on  the  banks  or  roadside  appear  to 
move  in  an  OPPOSITE  DIBECTION  ? 

'    The  relative  situation  of  the  trees  and  buildings  to  the  person,  and  to  each 


COMPOUND   MOTION.  85 

other,  is  actually  changed  by  the  motion  of  the  observer ;  but  the  mind,  in 
judging  of  the  real  change  in  place  by  the  difference  in  the  position  of  the 
objects  observed,  unconsciously  confounds  the  real  and  apparent  motion. 

5.  Why  will  a  tallow  candle  fired  from  a  gun  pierce  a  board,  or  target,  in  the  same 
manner  as  a  leaden  bullet  will,  under  the  Bame  circumstances  ? 

"When  a  candle  starts  from  the  breach  of  a  gun,  its  motion  is  gradually  in- 
creased, until  it  leaves  the  muzzle  at  a  high  velocity ;  and  when  it  reaches  the 
board,  or  target,  every  particle  of  matter  composing  it  is  in  a  state  of  great 
Telocity.  At  the  moment  of  contact,  the  particles  of  matter  composing  the 

.  target  are  at  rest ;  and  as  the  density  of  the  candle,  multiplied  by  the  velocity 
of  its  motion,  is  greater  than  the  density  of  the  target  at  rest,  the  greater  force 

•overcomes  the  weaker,  and  the  candle  breaks  through  and  pierces  a  hole  in 
the  board. 

6.  Why,  with  an  enormous  pressure  and  slow  motion,  can  you  not  force  a  candle 
through  a  board  ? 

Because  the  candle,  on  account  of  its  slow  motion,  does  not  possess  suffi- 
cient momentum  to  enable  the  density  of  its  particles  to  overcome  the  greater 
density  of  the  board ;  consequently  the  candle  itself  is  mashed,  instead  of 
piercing  the  board. 

7.  Why  will  a  large  ship,  moving  toward  a  wharf  with  a  motion  hardly  perceptible, 
crush  with  great  force  a  boat  intervening  ? 

Because  the  great  mass  and  weight  of  the  vessel  compensates  for  its  want 
of  velocity. 

8.  Why  can  a  person  safely  skate  with  great  rapidity  over  ice  which  would  not  bear 
his  weight  standing  quietly  ? 

Because  time  is  required  to  produce  a  fracture  of  the  ice ;  as  soon  as  the 
weight  of  the  skater  begins  to  act  upon  any  point,  the  ice,  supported  by  the 
water,  bends  slowly  under  him ;  but  if  the  skater's  velocity  be  great,  he 
passes  off  from  the  spot  which  was  loaded  before  the  bending  has  reached 
the  point  at  which  the  ice  would  break. 

9.  A  HEAVY  COACH  and  a  LIGHT  WAGON  came  in  collision  on  the  road.    A  suit  for 
damages  was  brought  by  the  proprietor  of  the  wagon.     How  was  it  shown  that  ONE  of  the 
VEHICLES  was  moving  at  an  UNSAFE  VELOCITY  ? 

On  trial,  the  persons  in  the  wagon  deposed  that  the  shock,  occasioned  by 
coming  in  contact,  was  so  great,  that  it  threw  them  over  the  head  of  their  horse; 
and  thus  lost  their  case  by  proving  that  the  faulty  velocity  was  their  own. 

10.  Why  did  the  FACT  that  they  were  THBOWN  over  the  HEAD  OF  THE  HOESK  by  coming 
in  contact  with  the  coach,  prove  that  their  velocity  was  GBEATEB  than  it  ought  to  have 
been? 

The  coach  stopped  the  wagon  by  contact  with  it,  but  the  bodies  of  the  per- 
sons in  the  wagon,  having  the  same  velocity  as  the  wagon,  and  not  fastened  to 
it,  continued  to  move  on.  Had  the  wagon  moved  slowly,  the  distance  to  which 
they  would  have  been  thrown  would  have  been  slight  To  cause  them  to 
be  thrown  as  far  as  over  the  head  of  the  horse,  would  require  a  great  velocity 
of  motion. 


86  WELLS'S  NATURAL  PHILOSOPHY. 

11.  When  TWO  PEBSONB  STRIKE  their  BEADS  together,  one  being  in  MOTION  and  the  other 
at  BEST,  why  are  both  equally  hurt? 

Because,  when  bodies  strike  each  other,  action  and  reaction  are  equal ;  the 
head  that  is  at  rest  returns  the  blow  with  equal  force  to  the  head  that 
strikes. 

12.  When  an  elastic  BALL  is  thrown  against  the  Bide  of  a  house  with  a  CEBTAIN  FOBCE, 
why  does  it  rebound  ? 

-  Because  the  side  of  the  house  resists  the  ball  with  the  same  force,  and  the 
ball  being  elastic,  rebounds. 

13.  When  the  SAME  BALL  is  thrown  against  a  PANE  of  GLASS  with  the  same  force,  it  goes 
through,  breaking  the  glass ;  why  does  it  not  rebound  as  before  ? 

Because  the  glass  has  not  sufficient  power  to  resist  the  full  force  of  the  ball : 
it  destroys  a  part  of  the  force  of  the  ball,  but  the  remainder  continuing  to  act, 
the  ball  goes  through,  shattering  the  glass. 

14.  Why  did  not  the  MAN  succeed  who  undertook  to  make  a  FAIB  WIND  for  his  PLEAS- 
TOE-BOAT,  by  erecting  an  IMMENSE  BELLOWS  in  the  STESN,  and  blowing  against  the  SAILS  ? 

Because  the  action  of  the  stream  of  wind  and  the  reaction  of  the  sails  were 
exactly  equal,  and,  consequently,  the  boat  remained  at  rest. 

15.  If  he  had  blown  in  a  CONTKABY  DIBECTION  from  the  sails,  instead  of  against  them, 
would  the  boat  have  moved  ? 

It  would,  with  the  same  force  that  the  air  issued  from  the  bellows-pipe. 

16.  Why  can  not  a  MAN  raise  himself  over  a  FENCE  by  pulling  upon  the  BTBAPB  of  his 

BOOTS? 

Because  the  action  of  the  force  exerted  by  the  muscles  of  his  arms  is  coun- 
teracted by  the  reaction  of  the  force,  or,  in  other  words,  the  resistance  of  his 
whole  body,  which  tends  to  keep  him  down. 

17.  Why  do  WATEB-DOGB  give  a  SEMI-BOTABY  MOVEMENT  to  free  themselves  from 
water  ? 

Because  in  this  way  a  centrifugal  force  is  generated,  which  causes  the  drops 
of  water  adherent  to  them  to  fly  off. 

18.  Why  is  the  COTTOSE  of  rivers  rarely  STBAIGHT,  but  SEBPENTINE  and  WINDING  f 
When,  from  any  obstruction,  the  river  is  obliged  to  bend,  the  centrifugal 

force  tends  to  throw  away  the  water  from  the  center  of  the  curvature,  so  that 
when  a  bend  has  once  commenced,  it  increases,  and  is  soon  followed  by  others. 
Thus,  for  instance,  the  water  being  thrown  by  any  cause  to  the  left  side,  it 
wears  that  part  into  a  curve,  or  elbow,  and,  by  its  centrifugal  force,  acts  con- 
stantly on  the  outside  of  the  bend,  until  the  rock,  or  higher  land,  resists  its 
gradual  progress ;  from  this  limit,  being  thrown  back  again,  it  wears  a  similar 
bend  to  the  right  hand,  and  after  that  another  to  the  left,  and  so  o»» 

19.  A  locomotive  passes  over  a  railroad,  200  miles  in  length,  in  5  hours ;  what  is  its 
Telocity  per  hour  ? 

20.  If  a  bird,  in  flying,  passes  over  a  distance  of  45  miles  in  an  hour,  what  is  its  ve- 
locity per  minute  ? 

SI.  The  flash  of  a  cannon  three  miles  off  was  seen,  and  in  14  seconds  afterirard  the 
sound  was  heard.  Row  many  feet  did  the  sound  travel  in  one  second  ? 


.-•      APPLICATION    OF   FORCE.   ':.*  '87 

22.  The  sun  is  95  millions  of  miles  from  the  earth,  atid  it  requiies  8J  minutes  for  its 
light  to  reach  the  earth  ;  with  what  velocity  per  second  does  light  move  ? 

23.  If  a  vessel  sail  90  miles  a  day  for  8  days,  how  far  will  it  sail  in  that  time  ? 

24   A  gentle  wind  is  observed  to  move  1,250  feet  in  15  minutes :  how  far  would  it  move 
in  2  hours,  allowing  5,000  feet  to  the  mile  ? 

.  25  What  distance  would  a  bird  flying  uniformly  at  the  velocity  of  CO  miles  per  hour, 
pass  over  in  12J  hours? 

•  26.  Suppose  light  to  move  at  the  rate  of  192,000  miles  in  a  second  of  time,  how  long  a 
time  will  elapse  in  the  passage  of  light  from  the  sun  to  the  earth,  the  distance  being  95 
millions  of  miles  ? 

27.  What  is  the  momentum  of  a  body  weighing  25  pounds  moving  with  the  velocity 
of  30  feet  per  second  ? 

28.  A  cannon-ball  weighing  520  pounds,  struck  a  wall  with  a  velocity  of  45  feet  per 
second :  what  was  its  momentum,  or  with  what  force  did  it  strike  ? 

29.  A  locomotive  and  train  of  cars  weighing  180  tons  (403,200  pounds),  and  moving  at 
the  rate  of  40  miles  per  hour,  came  in  collision  with  another  train  weighing  160  tons,  and 
moving  at  the  rate  of  25  miles  per  hour :  what  was  the  momentum,  or  force  of  collision  ? 

30.  A  stone  thrown  directly  at  an  object  from  a  locomotive,  moving  at  the  rate  of  3,520 
feet  per  minute,  was  2  seconds  in  the  air;  at  what  distance  beyond  the  object  did  it 
•trike  ? 


CHAPTER    VI. 

APPLICATION    OF    FORCE. 

what  are  the  174.  THE  principal  agents  from  whence  we 
J^polwfTfhe  obtain  power  for  practical  purposes,  are  MEN 
arts?  and  ANIMALS,  WATER,  WIND,  STEAM,  and 

GUNPOWDER. 

The  power  of  all  these  may  be  ultimately  resolved  into  some 
great*  nahiral  one  or  more  of  the  great  natural  forces,  or  primary  sources  of 
forces  are  these  power,  viz.,  vital  force,  producing  muscular  energy,  or  strength 
derived?1*  ^  iQ  man  and  animals;  gravitation,  causing  the  flow  of  water; 

heat  and  molecular  forces,  the  agents  producing  the  power  ex- 
hibited  by  wind,  steam,  and  gunpowder. 

Are  there  any  Magnetism  and  electricity  when  called  into  action,  and 
other  agents  of  capillary  attraction,  are  also  agents -of  power;  but  none  of 

these  are  capable,  as  yet,  of  being  used  to  any  great  extent 
for  the  production  of  motion. 

175.  Muscular  energy  in  men  and  animals 

Howismuscu-      ^  »* 

is  exerted  by  means  of  the  contraction  of  the 
fibers  which  constitute  the  muscles  of  the 


88  WELLS'S   NATURAL   PHILOSOPHY. 

body  ;  the  bones  of  the  body  facilitate  and  direct  the  ap- 
plication of  this  force. 

Beasts  of  prey  possess  the  greatest  amount  of  muscular  power ;  but  somo 
very  small  animals  possess  muscular  power  in  proportion  to  their  bulk,  in- 
comparably greater  than  the  largest  of  the  brute  creation.  A  flea,  considered 
relatively  to  its  size,  is  stronger  than  an  elephant,  or  a  lion. 
How  can  a  man  ^  man  can  exer^  *"s  greatest  active  strength  in  pulling  up- 
exert  his  great-  ward  from  his  feet,  because  the  strong  muscles  of  the  back, 
est  strength  ?  and  those  of  the  Upper  an(j  iower  extremities,  are  then  brought 
most  advantageously  into  action. 

The  comparative  effect  produced  in  the  different  methods  of  applying  the 
force  of  a  man,  may  be  indicated  as  follows :  in  the  action  of  turning  a  crank, 
or  handle,  his  force  may  be  represented  by  the  number  17 ;  in  working  a 
pump,  by  20 ;  in  pulling  downward,  as  in  ringing  a  bell,  by  39  ;  and  in  pull- 
ing upward  from  the  feet,  as  in  the  action  of  rowing,  by  41. 

what  is  the  176.  The  estimate  of  the  uniform  strength 
6\sre™gthdof  a  °f  an  ordinary  man,  for  the  performance  of  or- 
dinary daily  mechanical  labor  is,  that  he  can 
raise  a  weight  of  10  pounds  to  the  height  of  10  feet  once 
in  a  second,  and  continue  to  do  so  for  10  hours  in  the 
day. 

what  is  the         ^^'  ^ke  estimated  strength  of  a  horse  is, 
estimated          that  he  can  raise  a  weight  of  33,000  pounds 

strength  of  a  &  .  .*  01 

horse,   or   a     to  the  height  ot  one  foot  in  a  minute.     Such 

"horse-power?"  .  • 

a   measure    of   force    is    called    a    "  HORSE- 
POWER." 

The  strength  of  a  horse  is  considered  to  be  equal  to  that  of  five  men.  The 
average  strength  which  a  horse  can  exert  in  drawing  is  about  1600  pounds. 

whatiswater-  178.  WATER-POWER  is  the  power  obtained 
power?  kv  tke  actjon  Of  water  falling  perpendicularly, 
or  running  down  a  slope,  by  the  influence  of  gravity, 
what  is  the  179.  When  work  is  performed  by  any  agent, 
the  there  is  always  a  certain  weight  moved  over  a 
certain  space,  or  a  resistance  overcome  ;  the 
different  forces?  amount  of  work  performed,  therefore,  will  de- 
pend on  the  weight,  or  resistance  that  is  moved,  and  the 
space  over  which  it  is  moved.  For  comparing  different 
quantities  of  work,  done  by  any  force,  it  is  necessary  to 
have  some  standard ;  and  this  standard  is  the  power,  or 


APPLICATION    OF    FORCE.  5U 

labor,  expended  in  raising  a  pound  weight  one  foot  high, 
in  opposition  to  gravity. 

HOW  is  the  ef-  180.  The  effect  produced  by  a  moving  power 
iS?  power"ex-  '1S  alwavs  expressed  by  a  certain  weight  raised 
pressed?  a  certain  height. 

To  find,  therefore,  the  effect  of  a  moving  power,  or  to  find  the  power  ex- 
pended in  performing  a  certain  work,  we  have  the  following  rule:  — 

HOW  ma7  the  181-  Multiply  the  weight  of  the  body  moved 
SlntrSTbe  in  pounds  by  the  vertical  space  through  which 

ascertained? 


Thus,  for  example,  if  a  horse  draw  a  loaded  wagon,  with  a  force  by  which 
the  traces  are  stretched  to  as  great  a  degree  as  if  200  pounds  y  re  suspended 
vertically  from  them,  and  if  the  horse  thus  acting  draws  the  wagon  over  a 
space  of  100  feet,  the  mechanical  effect  produced  is  said  to  be  200  pounds 
raised  100  feet;  or,  what  is  the  same  thing,  20,000  pounds  raised  1  foot. 
"When  a  horse  draws  a  carriage,  the  work  he  performs  is  expended  in  over- 
coming the  resistance  of  friction  of  the  road  which  opposes  the  motion  of  tho 
carriage  ;  but  friction  increases  and  diminishes  as  the  weight  of  the  load  in- 
creases or  diminishes.  The  work  performed  will,  therefore,  be  estimated  by 
multiplying  the  total  resistance  of  friction,  as  expressed  in  pounds,  by  the 
space  over  which  the  carriage  is  moved. 

The  following  examples  will  illustrate  how  we  are  enabled, 
manner  of  esti-  by  the  above  rules,  to  calculate  the  amount  of  power  required 
mating  power?  ^o  perform  a  certain  amount  of  work:  —  Suppose  we  wish  to 
know  the  amount  of  horse-power  required  to  lift  224  pounds  of  coal  from  the 
bottom  of  a  mine  600,  feet  deep.  The  weight,  224,  multiplied  into  space 
moved  over,  600  feet,  equals  134,400,  the  amount  of  work  to  be  performed 
each  minute;  a  horse-power  equals  33,000  pounds  raised  1  foot  per  minute: 
therefore,  134,  400-r-33,000=r  4.07,  horse-power  required.  If  we  wish  to  per- 
form the  same  work  by  a  steam-engine,  we  would  order  an  engine  of  4.07 
horse-power,  and  the  engine-builder,  knowing  the  dimensions  of  the  parts  of 
an  engine  essential  to  give  one  horse-power,  can  build  an  engine  capable  of 
performing  the  requisite  work. 

Again.  Suppose  a  locomotive  to  move  a  train  of  cars,  on  a  level,  at  tho 
rate  of  30  miles  per  hour,  the  whole  weighing  25  tons,  with  a  constant  re- 
sistance from  friction  of  200  pounds,  what  is  the  horse-power  of  the  engine? 
30  miles  per  hour  equals  2,640  feet  per  minute  ;  this  space  multiplied  by  200 
pounds,  the  resistance  to  be  overcome,  equals  528,000,  the  work  to  be  done 
every  minute;  which,  divided  by  33,000  (one  horse-power),  equals  16,  the 
horse-power  of  the  locomotive. 

what  is  a  Dy-  182.  An  instrument  for  measuring  the  rela- 
namometer?  tive  g^ng^  of  men  and  animals,  and  also  of 
the  force  exerted  by  machinery,  is  called  a  DYNAMOMETER. 


90  WELLS'S   NATURAL   PHILOSOPHY. 

pI(J  59  Fig.  59  represents  one  of  the  most 

common   forms  of  the   dynamometer, 

'bCt^iF'''^!.?^—^, consisting  of  a  band  of  steel,  bent  in 

the  middle,  so  as  to  have  a  certain  de- 
gree of  flexibility.  To  the  expanded 
extremity  of  each  limb  is  fixed  an  arc 
of  iron,  which  passes  freely  through  an 
opening  in  the  other  limb,  and  terminates  outside  in  a  hook  or  ring.  One  of 
these  arcs  is  graduated,  and  represents  in  pounds  the  force  required  FlG  60> 
to  bring  the  two  limbs  nearer  together.  Thus,  if  a  horse  were  pulling 
a  rope  attached  to  a  body  which  he  had  to  move,  we  may  imagine  the 
rope  to  be  cut  at  a  certain  point,  and  the  two  ends  attached  to  the 
ends  of  the  arcs,  as  represented  in  Fig.  59 ;  the  force  of  traction  ex- 
erted by  the  animal  would  be  seen  by  the  greater  or  less  bringing 
together  of  the  ends  of  the  instrument. 

In  another  form  of  dynamometer,  Fig.  60,  which  is  also  used  as  a 
spring  balance  in  weighing,  the  force  is  measured  by  the  collapsing 
of  a  steel  spring,  contained  within  a  cylindrical  case.  The  construc- 
tion and  operation  of  this  instrument  will  be  easily  understood  from 
an  examination  of  the  figure. 

what  is  a  Ma-         183.  A  MACHINE  is  an  instrument,  or 
apparatus,  adapted  to  receive,  distribute, 
and  apply  motion  derived  from  some  external  force, 
in  such  a  way  as  to  produce  a  desired  result. 

A  steam-engine  and  a  water-wheel  are  examples  of  machines.  They  re- 
ceive the  power  of  steam  in  the  one  case,  and  the  power  of  falling  water  in 
the  other,  and  apply  it  for  locomotion,  sawing,  hammering,  etc. 

DO  we  produce  184.  A  MACHINE  can  not,  under  any  cir- 
u°sece  of7  mat  cumstances,  create  power,  or  increase  the 
chines?  quantity  of  power,  or  force,  applied  to  it. 

A  machine  will  enable  us  to  concentrate,  or  divide,  any  quantity  of  force 
which  we  may  possess,  but  they  no  more  increase  the  quantity  of  force  applied 
than  a  mill-pond  increases  the  quantity  of  water  flowing  in  the  stream.* 

Machines,  in  fact,  do  not  increase  an  applied  force,  but  they 
chines  in  reality  diminish  it,  or,  in  other  words,  no  machine  ever  transmits  the 
diminish  force  ?  whole  amount  of  force  imparte(i  to  it  by  the  moving  power; 
since  a  part  of  the  power  is  necessarily  expended  in  overcoming  the  inertia 
of  matter,  the  friction  of  the  machinery,  and  the  resistance  of  the  atmosphere. 

*  "  Power  is  always  a  product  of  nature.  God  has  not  vouchsafed  to  man  the  means  of 
its  primary  creation.  He  finds  it  in  the  moving  air  and  the  rapid  cataract ;  in  the  burn- 
ing coal  and  the  heaving  tide.  He  transfers  it  from  these  to  other  bodies,  and  renders  it 
the  obedient  servant  of  his  will— the  patient  drudge  which,  in  a  thousand  ways,  adminis- 
ters to  his  wants,  his  convenience,  and  his  luxuries,  and  enables  him  to  reserve  his  own 
energy  for  the  higher  purposes  of  the  development  of  his  mind  and  the  expression  of  his 
thoughts."— Prof.  Henry. 


APPLICATION   OF   FORCE.  91 

• 

is  Perpetual  185.  PERPETUAL  MOTION,  or  the  construc- 
chSrVpoTst-  tion  °f  machines  which  shall  produce  power 
ble?  sufficient  to  keep  themselves  in  motion  con- 

tinually, is,  therefore,  an  impossibility,  since  no  combi- 
nation of  machinery  can  create,  or  increase,  the  quantity 
of  power  applied,  or  even  preserve  it  without  diminution. 

What  exam  le  In  nature  we  liave  an  example  of  continued  and  undimin- 
of  continued  ished  motion  in  the  revolution  of  the  earth  upon  its  axis,  and 
we  in  natuTe?  of  the  Planets  around  the  sun.  These  bodies  have  been  mov- 
ing with  undiminished  velocity  for  ages  past,  and,  unless  pre- 
vented by  the  agency  which  created  them,  will  continue  so  to  do  for  ages  to  come. 

186.  We  derive  advantages  from  machines 

How  do  we  de- 

rive  advantages    in  three  different  ways:  1st,  from  the  addi- 

from  machines  ?  |      7 

tions  they  make  to  human  power  ;  2d,  from 
the  economy  they  produce  of  human  time  ;  3d,  from  the 
conversion  of  substances  apparently  worthless  and  com- 
mon into  valuable  products. 

HOW  do  ma-  187.  Machines  make  additions  to  human 
additi8on8ma^o  power,  because  they  enable  us  to  use  the 
human  power?  p0wer  Of  natural  agents,  as  wind,  water,  steam. 
They  also  enable  us  to  use  animal  power  with  greater  ef- 
fect, as  when  we  move  an  object  easily  with  a  lever,  which 
we  could  not  with  the  unaided  hand. 
HOW  do  ma-  188.  Machines  produce  economy  of  human 
econompyrof  hue-  time,  because  they  accomplish  with  rapidity 
man  time?  what  would  require  the  hand  unaided  much 
time  to  perform. 

A  machine  turns  a  gun-stock  in  a  few  minutes ;  to  shape  it  by  hand  would 
be  the  work  of  hours. 

189.  Machines   convert  objects  apparently 

How   do   ma-  11-^  i       i  i  i  i 

chines  convert    worthless  into  valuable  products,  because  by 

worthless    ob-         .      .  .,.",. 

jects  into  vai-     their  qreat  power,  economy,  and  r  avidity  of 

uable  products?  . .          ,,  ,       -,  A&\  L«      1 

action,  they  make  it  profitable  to  use  objects 
for  manufacturing  •  purposes  which  it  would  be  unprofit- 
able or  impossible  to  use  if  they  were  to  be  manufactured 
by  hand. 

Without  machines,  iron  could  not  be  forged  into  shafts  for  gigantic  engines ; 
fibers  could  not  be  twisted  into  cables;  granite,  in  large  masses,  could  not  b0 
transported  from  the  quarries. 


92  WELLS'S  NATURAL  PHILOSOPHY. 

Define  power,  190.  In  machinery,  we  designate  the  mov- 
wSffing  point,  ing  *orce  as  the  POWER  ;  the  resistance  to  be 
macSery.  to  overcome,  whatever  may  be  its  nature,  as  the 
WEIGHT  ;  and  the  part  of  the  machine  im- 
mediately applied  to  the  resistance  to  be  overcome,  as  the 
WORKING  POINT. 

what  is  the  191.  The  great  general  advantage  that  we 
fdvantaggeenerS  obtain  from  machinery  is,  that  it  enables  us 
machinery?  to  exchange  time  and  space  for  power. 

Thus,  if  a  man  could  raise  to  a  certain  height  two  hundred  pounds  in  one 
minute,  with  the  utmost  exertion  of  his  strength,  no  arrangement  of  machinery 
could  enable  him  unaided  to  raise  2,000  pounds  in  the  same  time.  If  he  de- 
sired to  elevate  this  weight,  he  would  be  obliged  to  divide  it  into  ten  equal 
parts,  and  raise  each  part  separately,  consuming  ten  times  the  time  required 
for  lifting  200  pounds.  The  application  of  machinery  would  enable  him  to 
raise  the  whole  mass  at  once,  but  would  not  decrease  the  time  occupied  in 
doing  it,  which  would  still  be  ten  minutes. 

Again.  A  boy  who  can  not  exert  a  force  of  fifty  pounds  may,  by  means 
of  a  claw-hammer,  draw  out  a  nail  which  would  support  the  weight  of  half  a 
ton.  It  may  seem  that  the  use  of  the  hammer  in  this  case  creates  power, 
but  it  does  not,  since  the  hand  of  the  boy  is  required  to  move  through  per- 
haps one  foot  of  space  to  make  the  nail  rise  one  quari&r  of  an  inch.  But  it  has 
"been  already  shown  that  the  force  of  a  small  body  moving  with  great  velocity 
may  equal  the  force  of  a  large  body  with  a  slight  velocity.  On  the  same  prin-r 
ciple,  the  small  weight,  or  power,  exerted  by  the  boy  on  the  end  of  the  ham- 
mer handle,  moving  through  a  large  space  with  an  increased  velocity,  ac- 
quires sufficient  momentum  to  overcome  the  grdat  resistance  of  the  naiL 

In  both  of  these  examples  space  and  time  are  exchanged  for  power. 

192.  The  mechanical  force,  or  momentum,  of  a  body,  is  as- 
How  is  the  me- 
chanical effect      certained  by  multiplying  its  weight  by  the  space  through 

d°"      which  it  moves  in'  a  given  time,  that  is  to  say,  by  its^  velocity. 


The  mechanical  force,  or  momentum,  of  a  power  may  also  be 
found,  by  multiplying  the  power,  or  its  equivalent  weight,  by  its  velocity. 

what  is  the  193.  The  power,  multiplied  by  the  space 
bar7umf  o?uaii  through  which  it  moves  in  a  vertical  direction, 
machines?  -g  equa}  ^o  the  weight  multiplied  by  the  space 
through  which  it  moves  in  a  vertical  direction. 

This  is  the  general  law  which  determines  the  equilibrium  of  all  machines. 

194.  The  power  will  overcome  the  resistance 

Undor    -what  »  v  -.  •  «ni          i 

conditions  will    of  the  weight,  and  motion  will  take  place  m  a 

motion      take  .  .  i  vi  i  •    •  /•  ji 

place  in  a  ma-    machine,  when  the  product  arising  trom  the 
power  multiplied  by  the  space  through  which 


THE   ELEMENTS   OF   MACHINERY.  93 

it  moves  in  a  vertical  direction,  is  greater  than  the  pro- 
duct arising  from  the  weight  multiplied  by  the  space 
through  which  it  moves  in  a  vertical  direction. 

Practical  men  express  the  principle  of  equilibrium  in  ma- 
What  is  meant 

by  the  ex-  chinery  by  sajang  "that  what  is  gained  in  power  is  lost  in 
presFion  Pow"er  time."  Thus,  if  a  small  power  acts  against  a  great  resistance, 
the  expense  of  the  motion  of  the  latter  will  be  just  as  much  slower  than  that 
time  ?  of  the  power,  as  the  resistance,  or  weight,  is  greater  than  the 

power ;  or  if  one  pound  be  required  to  overcome  the  resistance  of  two  pounds, 
the  one  pound  must  move  over  two  feet  in  the  same  time  that  the  resistance, 
two  pounds,  requires  to  move  over  one. 

SECTION    I. 

THE     ELEMENTS     OF     MACHINERY. 

195.  All  machines,  no  matter  how  complex 

How  many  sim-  .     .     ,    .  ,  ..  i 

pie    machines   and  intricate  their  construction,  may  be  re- 
duced to  one  or  more  of  «eix  simple  machines, 
or  elements,  which  we  call  the  "  MECHANICAL  POWERS." 

196.  They  are  the  LEVER,  the  WHEEL  and 

Enumerate  the  .1  T  _ 

six  elementary       AXLE,   tllC   PULLEY,  the  INCLINED  PLANE,  the 

machines.  ,      ..        ~ 

WEDGE,  and  the  SCREW. 

These  simple  Machines  may  be  further  reduced  to  three— the  lever,  the 
pulley,  and  the  inclined  plane ;  since  the  wheel  and  axle,  the  screw  and  the 
wedge,  may  be  regarded  as  modifications  of  them. 

The  name  "  mechanical  powers"  which  has  been  applied  to  the  six  ele- 
mentary machines,  is  unfortunate,  since  it  serves  to  convey  an  idea  that  they 
are  really  powers,  when  in  fact  they  possess  no  power  in  themselves,  and  are 
only  instruments  for  the  application  of  power. 

what  is  a  197-  A  LEVER  consists  of  a  solid  bar,  straight 

Lever?  or  ^en^  turning  upon  a  pivot,  prop,  or  axis. 

what  are  the  198.  The  ARMS  of  the  lever  are  those  parts 
Arms  of  a  Le-  Of  ^Q  j^,.  extending  on  each  side  of  the 

axis. 

what  is  the        199.  The  FULCRUM,  or  prop,  is  the  name 
applied  to  the  axis,  or  point  of  support. 

200.  Levers  are  divided  into  three  kinds,  or 

How    many  ..  i  •    i         i 

kinds  of  levers    classes,  according  to  the  position  which  the 
fulcrum  has  in  relation  to  the  power  and  the 
weight. 


94 


WELLS'S   NATURAL   PHILOSOPHY". 


What  are  the 
relative  posi- 
tions of  the 
power,  fulcrum 
and  weight  in 
the  three  kinds 
of  levers  ? 


FIG.  61. 

/ 


What  are  ex- 
amples of  le- 
vers of  the 
first  class  ? 


U 


201,  In  the  first  class  the  fulcrum  is  be-' 
tween  the  power  and  the  weight  ;  in  the  sec- 
ond class,  the  fulcrum  is  at  one  end  of  the 
lever,  and  the  weight  is  between  the  fulcrum 
and  the  power  ;  in  the  third  class,  the  fulcrum 

is  at  one  end  of  the  lever,  and  the  power  is  between  the 

fulcrum  and  the  weight. 

Fig.  61  represents  the  three  classes  of 
levers,  numbered  in  their  order,  1.  2,  3. 
P  is  the  power,  "W  the  weight,  and  F  the 
fulcrum. 

A  crowbar  applied  to 
elevate  a  stone,  is  an  ex- 
ample of  a  lever  of  the 
first  kind.  In  Fig.  62, 
which  represents  a  lever  of  this  class,  a 
indicates  the  fulcrum  which  suppports  the 
bar,  b  the  power  applied  by  Jhe  hand  at 
the  end  of  the  longest  arm,  and  c  the 
weight,  or  stone,  raised  at  the  end  of  the 

short  arm.     A  poker  applied  to  stir  up  the  fuel  of  a  grate  is  a  lever  of  the 

first  class,  the  fulcrum  being  the 
bars  of  the  grate ;  the  break,  or 
or  handle  of  a  pump,  is  also  a  fa- 
miliar example.  Scissors,  pin- 
cers, etc.,  are  composed  of  two 
levers  of  the  first  kind,  the  ful- 
crum being  the  joint,  or  pivot, 
and  the  weight  the  resistance 
The  power  of  the  fingers  is  applied 


FIG.  62. 


What  ia  the 
law  of  equili- 
brium of  the 
lever  ? 


of  the  substance  to  be  cut,  or  seized, 
at  the  other  end  of  the  levers. 

202.  A  lever  will  be  in  equilibrium,  when 
the  power  and  the  weight  are  to  each  other 
inversely  as  their  distances  from  the  fulcrum. 

Thus,  if  in  a  lever  of  the  first  class  the  power  and  the  weight  are  equal, 
and  are  required  to  exactly  balance  each  other,  they  must  be  placed  at 
e^ual  distances  from  the  fulcrum.  If  the  power  is  only  half  the  weight,  it 
must  be  at  double  the  distance  from,  the  fulcrum;  if  one  third  of  the 
weight,  three  times  the  distance.  If  we  suppose,  in  Fig.  62,  c  to  represent 
a  weight  of  300  pounds,  placed  two  feet  from  the  fulcrum  a,  and  6  a  power 
of  100  pounds  placed  six  feet  from  a,  then  c  and  b  will  be  in  equilibrium, 
for  (300X2) -(100X6). 

203.  When  the  weight  and  lengths  of  the  two.  arms 


THE   ELEMENTS   OF   MACHINERY. 


95 


Weight,  and 
the  length  of 
the  anus  of  a 
lever  being 
given,  how  we 
find  the  equiv- 
alent power  ? 


What  are  ex- 
amples of  le- 
vers of  the 
second  class  ? 


of  a  lever  are  given,  the  power  requisite  to. 
balance  the  weight  may  be  ascertained,  by 
dividing  the  product  of  the  weight  multiplied 
into  its  distance  from  the  fulcrum,  by  the  dis- 
tance of  the  power  from  the  fulcrum. 

204.  Cork,  or  lemon-squeezers,  Fig.  G37  are  examples  of 
the  levers  of  the  second  class,  which  have  the  fulcrum  at  one 
end,  and  the  weight,  or  resistance  to  be  overcome,  between 
the  fulcrum  and  the  power.     An  oar  is  a  lever  of  the  second 
class,  in  which  the  reaction  of  the  water  against  the  blade  is  the  fulcrum,  the 
63  boat  the  weight,  arid  the  hand  of 

the  boatman  the  power.  A  door 
moved  on  its  hinges  is  another 
example.  A  wheel-barrow  is  a 
lever  of  the  second  class,  the  ful- 
crum being  the  point  at  which  the- 
wheel  presses  upon  the  ground, 
the  barrow  and  its  load  the  weight, 
and  the  hands  the  power.  Nut- 
crackers are  two  levers  of  the  second  class,  the  hinge  which  unites  them  being 
the  fulcrum,  the  resistance  of  the  shell  placed  between  them  the  weight,  and 
the  hand  the  power. 

What  are  ex-          205>  A  ^[r  of  sugar-tongs  rep-  Fia  64> 

ampies  of  le-     resents  a  lever  of  the  third  class, 

third  cifss  ?he  in  which  the  P°wer  is  applied  be- 
tween the  fulcrum  and  the  resist- 
ance, or  weight.  In  Fig.  64,  the  fulcrum  is  at  a, 
the  resistance  is  the  piece  of  sugar  to  be  lifted  at 
&,  and  the  power  is  the  fingers  applied  at  c. 
When  a  man  raises  a  ladder  against  a  wall,  he 
employs  a  lever  of  the  third  class ;  the  fulcrum 
being  the  foot  of  the  ladder  resting  upon  the 
ground,  the  power  being  the  hands  applied  to 
raise  it,  and  the  resistance  being  the  weight  of  the  ladder. 

206.  In  levers  of  the  third  class,  the  power, 
being  between  the  fulcrum  and  the  weight, 
will  be  at  a  less  distance  from  the  fulcrum  than 
the  weight  ;  and,  consequently,  in  this  form 

of  lever  the  power  must  be   always   greater  than   the 

weight. 

Thus  (in  No.  3,  Fig.  61),  if  the  length  from  the  point  where  the  weight,  W, 
is  suspended  to  F  be  three  times  the  length  of  P  F,  then  a  weight  of  100 
pounds  suspended  at  W  will  require  a  power  of  300  applied  at  P  to  sustain  it. 


What  is  the  re- 
lation between 
the  power  and 
the  weight  in 
levers  of  the 
third  class? 


96 


WELLS'S   NATURAL   PHILOSOPHY. 


FiG.  65. 


Owing  to  its  mechanical  disadvantages,  this  class  of  levers 
circumstances  ^s  ral>ely  used,  except  where  a  quick  motion  is  required,  rather 
do  we  employ  than  great  force.  The  most  striking  examples  of  levers  of  tho 
third  class  ?  third  class  are  found  in  the  animal  kingdom.  The  limbs  of 
animals  are  generally  levers  of  this  description.  The  socket 
of  the  bone,  a,  Fig.  65,  is  the  fulcrum ;  a  strong  muscle  attached  to  the  bone 

near  the  socket  c,  and  extend- 
ing to  d,  is  the  power ;  and  the 
weight  of  the  limb,  together 
with  whatever  resistance,  iv,  is 
opposed  to  its  motion,  is  the 
weight.  A  very  slight  con- 
traction of  the  muscle  in  this 
case  gives  considerable  motion 
to  the  limb. 

The  leg  and  claws  of  a  bird, 
are  examples  of  the  third  class 
of  levers,  the  whole  arrange- 
ment being  admirably  adapted  to  the  wants  of  the  animal.  "When  a  bird  rests 
upon  a  perch,  its  body  constitutes  the  weight,  the  muscles  of  the  leg  the 
power,  and  the  perch  the  fulcrum.  Now,  tli3  greater  the  weight  of  the  body, 
the  more  strain  it  exerts  upon  the  muscles  of  the  claws,  which,  in  turn,  grasp 
the  perch  more  firmly :  consequently,  a  bird  sits  upon  its  perch  with  the 
greatest  ease,  and  never  falls  off  in  sleeping,  since  the  weight  of  the  body  is 
instrumental  in  sustaining  it. 

207.  A  COMPOUND  LEVER  is  a  combination 

What     is      a  _ 

compound  LC-     of  several  simple  levers,  so  arranged  that  the 
shorter  arm  of  one  may  act  upon  the  longer 
arm  of  another.     In  this  way,  the  power  of  a  small  force 
in  overcoming  a  large  resistance  is  greatly  multiplied. 

FIG.  66. 


6 


An  arrangement  of  compound  levers  is  shown  in  Fig.  66.  Here,  by  means 
of  three  simple  levers,  1  pound  may  be  made  to  balance  1000;  for  if  the  long 
arm  of  each  of  the  levers  is  ten  times  the  length  of  the  short  one,  1  pound 
at  the  end  of  the  first  one  will  exert  a  force  of  10  pounds  upon  the  end  of  the 
second  one,  which  will  in  turn  exert  ten  times  that  amount,  or  100  pounds, 
upon  the  end  of  the  third  one,  which  will  balance  ten  times  that  amount,  or 
1000  pounds,  at  the  other  extremity.  . 


THE    ELEMENTS   OF    MACHINERY. 


97 


What  arc  the 
disadvantages 
of  a  compound 
lever? 


Describe  the 
common  steel- 
yard. 


208.  The  disadvantage  of  a  compound  lever 
is,  that  its  exercise  is  limited  to  a  very  small 
space. 

209.  The  different  varieties  of  weighing  machines  are  varie- 
ties or  combinations  of  levers.     The  common  steel-yard  is  a 
lever  of  unequal  arms,  belonging  to  the  first  class.     It  consists 

of  a  bar  (Fig.  67)  marked  with  notches  to  indicate  pounds  and  ounces,  and  a 
weight  which  is  movable  along  the  notches.  The  bar  is  furnished  with  three 
hooks,  or  rings,  on  the  largest  of  which  the  article  to  be  weighed  is  always 
hung.  The  other  hooks  serve  to  support  the  instrument  when  it  is  in  use, 
and  the  pivot  by  which  they  are  attached  to  the  bar  serves  as  the  fulcrum. 
The  weight,  Q,  sliding  upon  the  bar,  balances  the  article,  P,  which  is  to  be 
weighed,  it  being  evident  that  a  pound  weight  at  D  will  balance  as  many 
pounds  at  P  as  the  distance  A  C  is  contained  in  the  space  D  C. 


It  may  happen  that  when  the  weight  Q  is  moved  to  the  last  notch  upon  the 
bar  B  C,  that  the  article  P  will  still  preponderate.  In  this  case,  the  steel-yard 
is  held  by  the  hook  or  ring  nearer  to  A,  which  hangs  down  in  the  figure,  and 
the  steel-yard  turned  over,  it  being  furnished  with  two  sets  of  notches  on 
opposite  sides  of  the  bar.  By  this  means  the  distance  of  P,  the  article  weighed, 
from  the  fulcrum  is  diminished,  and  the  weight  Q,  at  the  given  distance  upon 
the  opposite  side  of  the  fulcrum,  will  balance  a  proportionally  greater  resist- 
ance, or  weight. 

Describe    the          210'  Tlie  Ordmai7  balance  is  a  lever  of  the  first  class,  with 
ordinary   bal-      equal  arms,  in  which  the  power  and  the  weight  are  neces- 
sarily equal.    Fig.  68  shows  the  common  form.     The  fulcrum 
or  axis,  is  made  wedge-like,  with  a  sharp  knife-like  edge,  and  rests  upon  a 


98 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  68.  surface    of  hardened  steel,   or 

agate,  in  order  that  the  beam 
may  turn  easily.  The  scale- 
pans  are  suspended  by  chains 
from  points  precisely  at  equal 
distances  from  the  fulcrum, 
and  being  themselves  adjusted 
so  as  to  have  precisely  equal 
weights,  the  two  sides  will  perfectly  balance  when  the  pans  are  empty. 

211.  If  the  two  arms  of  a  scale-beam  be  not  of  perfectly 
equal  length,  a  smaller  weight  at  the  end  of  the  larger  arm 
will  balance  a  greater  weight  at  the  end  of  the  shorter.  An 
excess  of  half  an  inch  in  the  length  of  the  arm  of  the  beam, 
to  which  merchandise  is  attached,  where  the  arm  should  be 
eight  inches  long,  would  cheat  the  buyer  exactly  one  ounce  in  every  pound: 
This  fraud,  if  suspected,  might  be  detected  instantly,  by  transposing  the 
weight  and  the  article  balanced ;  the  lightest  would  then  be  at  the  end  of 
the  short  arm,  and  would  appear  lighter  than  it  actually  is. 


Under  what 
circumstances 
will  a  balance 
indicate  false 
weights  ? 


what  is  the  212.  Platform  scales,  and  scales  intended 
for  weighing  hay,  etc.,  are  usually  compound 
levers,  and  are  constructed  in  very  various 

forms,  but  all  depend  on  the  principles  above  explained. 

Fig.  69  represents  one  of  the  varieties,  and  Fig.  70  a  sec- 

PiG.  70.  - 


THE   ELEMENTS   OF   MACHINERY,  99 

tion  of  the  same,  showing  the  arrangement  and  combination 
of  the  levers. 

213.  "When  a  lever  is  applied  to  raise  a  weight  or  overcome 
stancesCiriimit      a  resistance,  the  space  through  which  it  acts  at  any  one  time 
the   utility  of     jg  small,  and  the  work  must  be  accomplished  by  a  succession 

of  short  and  intermitting  efforts.  These  circumstances,  there- 
fore, limit  the  utility  of  the  common  lever,  and  restrict  its  use  to  those  cases 
only  in  which  weights  are  required  to  be  raised  through  small  spaces. 

214.  When,  however,  a  continuous  motion  is  required,  as  in 
How  is  contin- 

uous motion  raising  ore  from  a  mine,  or  in  lifting  the  anchor  of  a  ship, 
obtained?  jn  or(jer  to  remove  the  intermitting  action  of  the  lever,  and 

render  it  continual,  we  employ  the  simple  machine  known  as  the  wheel  and 
axle,  which  is  only  another  form  of  the  lever,  in  which  the  power  is  made  to 
act  without  intermission. 

215.  The  form  of  the  simple  machine  de- 
wheei     and    nominated  the  WHEEL  and  AXLE,  consists  of 

a  cylinder,  termed  an  axle,;  revolving  on  an 
axis,  and  having  a  wheel  of  larger  diameter  immovably  at- 
tached to  it,  so  that  the  two  revolve  with  a  common  motion. 

' 


Describe     the  Fia.  71. 

action  of  the    .  the  axle  with   a  wheel  im- 


and  movabl7  attached  to  it,  and 
the  wheel  turning  on  pivots 
inserted  into  the  ends  of  the  axle.  Around 
this  axle  is  wound  a  rope,  to  which  is  at- 
tached the  weight  W,  and  around  the  wheel 
is  another  rope,  to  which  the  power,  P,  is 
applied.  It  is  evident  that  one  turn  of  the 
wheel  will  unwind  as  much  mote  rope  from 
the  wheel  than  it  winds  on  the  axle,  as  its 
circumference  is  greater.  The  power,  P,  will  therefore  pass  over  a  much  greater 
space  than  the  weight  W.  The  weight  on  the  axle,  which  may  be  considered 
as  acting  on  the  short  arm  of  a  lever  which  is  the  radius*  of  the  axle,  may 
be  much  heavier  than  the  power  which  acts  at  the  long  arm  of  a  lever,  which 
is  the  radius  of  the  wheel. 

Hence  the  advantage  gained  in  the  wheel  and  axle  is  equal  to  the  number 
of  times  that  the  radius  of  the  axle  is  contained  in  the  radius  of  the  wheel, 
and  to  estimate  the  mechanical  advantage  gained  by  the  wheel  and  axle,  we 
have  the  following  rule  : 

now  do  we         216.  The  power  is  to  the  weight,  as  the 
adr!Tntaege  *o?     diameter  of  the  wheel  is  to  the  diameter  of 

the  wheel  and        ,r  _    ,-.1^ 

axie?  the  axle. 


*  The  radius  of  a  wheel,  or  cylinder,  is  its  semi-diameter,  or  a  line  drawn  from  its  cen- 
ter to  its  circumference,    The  spoke  of  a  carriage  wheel  represents  its  radius. 


LOO 


WELLS'S   NATURAL   PHILOSOPHY. 


Fig.  72  represents  a  section  of  the  wheel  and  axle,  showing  the  radius 
of  the  axle,  b  c,  and  the  radius  of  the  wheel,  a  c.     The  two  being  in  a 


straight  line,  the  weights  hanging  in  opposition  are 
always  as  if  they  were  connected  by  a  horizontal  lever, 
a  c  ft,  turning  on  a  fulcrum  at  c.  If  the  radius  of  the- 
wheel,  or  the  length  of  the  longer  arm  of  the  lever,  a  c, 
bo  24  inches,  and  the  radius  of  the  axle,  or  the  length 
of  the  shorter  arm,  c  6,  be  3  inches,  then  the  advantage 
gained  would  be  24-5-3  =  8,  and  a  power  of  100  pounds 
applied  to  the  wheel  would  balance  a  weight  of  800  ap- 
plied to  the  axle. 

How  do  we  a  -  21>7'  The  metbods  of  applying  power 
ply  power  iu  in  the  wheel  and  axle  are  very  various, 


FIG.  72. 


Q 


it  not  being  essential  that  the  power  should  be  applied  by  a 
rope.     The  axle  is  sometimes  placed  in  a  vertical  or  upright 
position,  and  the  power  applied  by  means  of  levers,  or  bars,  inserted  into  holes 
FiG.  73.  in  one  end  of  the  axle.     A  capstan  of  a  ship,  Fig. 

73,  is  an  example  of  this. 

In  the  windlass,  a  handle,  or  winch,  is  sub- 
stituted in  the  place  of  a  wheel.  (See  Fig.  74.) 
In  this  case,  the  advantage  gained  is  equal  to 
the  number  of  times  that  the  length  of  handle  is 
greater  than  the  radius  of  the  axle.  Thus,  if  the 
handle  is  20  inches  and  the  radius  of  the  axle 
is  2  inches,  then  the  advantage  would  be  10,  and 
a  power  of  50  pounds  applied  at  the  handle  would 
just  raise  a  weight  of  10  time's  50,  or  500  pounds. 

"When  a  weight,  or  resistance,  of  comparatively  great  amount  is  to  be  raised 
by  a  very  small  power,  by  means  of  the  simple  wheel  and  axle,  either  of  two 
inconveniences  would  ensue ;  either  the  diameter  of  the  axle  would  become 
too  small  to  support  the  weight,  or  the  diameter  of  the  wheel  would  become 
so  great  as  to  be  unwieldy.  This  has  been  remedied  by  a  very  simple  ar- 
FiG.  74.  rangement,  called  the  double  axle,  Fig.  74. 

The  axle  of  the  windlass  here  consists  of 
two  parts  of  unequal  diameters,  and  the 
rope  winds  around  them  in  different  direc- 
tions ;  therefore,  every  turn  of  the  wind- 
lass, or  handle,  winds  up  a  portion  equal 
to  the  circumference  of  the  one,  but  un- 
winds a  portion  equal  to  the  circumference 
of  the  other,  and  if  the  two  be  nearly  equal, 
the  weight  moves  very  slow.  If  the  weight 
rise  1  inch  while  the  handle  describes  100 
inches,  1  pound  at  the  handle  will  balance  100  attached  to  the  rope. 

In  this  arrangement  space  and  time  are  exchanged  for  power  in  a  most 
convenient  manner. 


X 


THE   ELEMENTS   OF   MACHINERY. 


101 


What    ia  the 
most  frequent 
method   of 
transmitting 
motion  through 
a   combination 
of  wheels? 


FIG.  75. 


When  great  power  is  required,  wheels  and  axles  may  be  combined  to- 
gether in  a  manner  similar  to  that  of  the  compound  lever  already  explained 
(§  207).  By  such  a  combination  we  gain  the  advantage  .of  using  a  very  large 
wheel  with  a  small  axle,  without  their  inconveniences. 

218.  The  most   frequent  method  of  transmitting  motion 
through  a  combination  of  wheels,  is  by  the  construction  of 
teeth  upon  their  circumference,  so  that  the  teeth  of  each 
wheel  falling  between  those  of  the  other,  the  one  necessarily 
pushes  forward  the  other.     When  teeth  are  thus  affixed  to 
the  circumference  of  a  wheel,  they  are  termed  cogs-  upon  an 

axle,  they  are  termed  leaves,  while  the  axle  itself  is  called  a  pinion. 

Fig.  75  represents  a  combination 
of  wheels  and  axles  for  the  trans- 
mission of  power.  If  the  teeth  on 
the  axle  of  the  wheel  c  act  on  six 
times  the  number  of  teeth  on  the 
circumference  of  the  second  wheel, 
the  second  will  turn  only  once  for 
every  six  turns  of  the  first.  In  the 
same  manner  the  second  wheel,  by 
turning  six  times,  turns  the  third 
wheel  once ;  consequently,  if  the  proportion  between  the  wheels  and  their 
axles  be  preserved  in  all  three,  the  third  turns  once,  the  second  six  times, 
and  the  first  thirty-six  times.  Now,  as  the  wheel  and  axle  act  in  all  respects 
like  a  simple  lever,  and  a  combination  of  wheels  and  axles  as  a  combina- 
tion of  levers,  there  is  no  difficulty  in  understanding  how  a  mechanical  ad- 
vantage is  gained  by  this  contrivance.  The  power  is  to  the  weight  as  the 
product  of  the  diameter  of  all  the  axles  is  to  the  product  of  the  diameter  of 
all  the  wheels.  Thus,  if  the  diameter  of  all  the  axles  be  expressed  by  the 
numbers  2,  3,  and  4,  and  the  diameters  of  the  wheels,  c,/  and  g,  be  expressed 
by  the  numbers  20,  25,  and  30,  then  power  will  be  to  the  weight  as  2X3X 
4=24,  is  to  20X25X30  =  15,000  ;— or  a  power  of  24  at  the  first  wheel  will 
balance  15,000  at  the  axle  of  the  last  wheel 

219.  One  of  the  most  familiar  instances  of  combined  wheel- 
work  is  exhibited  in  clocks  and  watches.     One  turn  of  the  axle 
on  which  the  watch-key  is  fixed,  is  rendered  equivalent,  by  a 
train  of  wheel-work,  to  about  400  turns,  or  beats,  of  the  bal 
ance- wheel ;  and  thus  the  exertion,  during  a  few  seconds,  of 

the  hand  which  winds  up,  gives  motion  for  twenty-four,  or  thirty  hours.    By 


"What  are  fa- 
miliar illustra- 
tions of  com- 
pound wheel- 
work? 


FIG.  77. 


FlG  76  increasing  the  number  of  wheels, 

time-pieces  are  made  which  go  for 
a  year,  or  a  greater  length  of  time. 
Wheels  may  be  connected  and 
motion  communicated  from  one  to 
the  other,  by  bands,  or  belts,  as  well 
as  by  teeth.    This  principle  is  seen  in  the  spinning-wheel  and  common 
turning-lathe.     A  spinning-wheel,  as  a  c,  Fig.  76,  of  thirty  inches  in  circum- 


102 


WELLS'S   NATURAL   PHILOSOPHY. 


ference,  turns  by  its  band  a  smaller  wheel,  or  spindle,  b,  of  half  an  inch,  sixty 
times  for  every  revolution  of  a  c. 

"When  the  wheeHs  intended  to  revolve  in  the  same  direction  with  the  one 

from  which  it  receives  its  motion,  the*  band  is  attached  as  in  Fig.  76;  but 

•when  it  is  to  revolve  in  a  contrary  direction,  the  band  is  crossed,  as  in  Fig.  77. 

In  many  wheels  power  is  communicated  by  means  of  a  weight  applied  to 

the  circumference. 

In  the  tread-mill  (Fig.  78)  a  number  of  persons 
stepping  upon  the  circumference  of  a  wheel  cause 
it  to  revolve.  Similar  machines  are  often  adopted  in 
ferry-boats,  moved  by  horses,  and  called  "horse- 
boats." 

In  most  water-wheels,  power  is  obtained  by  the 
action  of  water  applied  to  the  circumference  of  the 
wheel,  which  is  caused  to  revolve,  either  through  the 
weight,  or  pressure  of  the  water,  or  by  both  conjointly. 


FIG.  78. 


What     is 
Pulley? 


How  many 
kinds  of  pul- 
leys are  there  ? 

What  is  a  fix- 
ed pulley? 


Describe  the 
working  and 
advantage  of 
the  fixed  pulley. 

FIG.  80. 

S    X 


220.  The  PULLEY  is  a  small  wheel  fixed  in 
a  block,  and  turning  on  an  axis,  by  means  of 
a  cord,  which  runs  in  a  groove  formed  on  the  edge  of  the 
wheel. 

This  simple  machine  is  represented  hi  Fig.  79.  •*  FIG.  79. 

220.  Pulleys  are  of  two  kinds  ; 
— fixed  and  movable. 

221.  By  a  fixed    pulley  we 
mean  one  that   merely  revolves 

on  its  axis,  but  does  not  change  its  place. 

Figs.  79  and  80  are  illustrations  of  fixed 
pulleys.     In  Fig.  80,  €  is  a  small  wheel  turning  upon  its 
axis,  around  which  a  cord  passes,  having  at  one  end  the 
power  P,  and  at  the  other,  the  resistance,  or  weight,  W.     It 
is  evident  that  by  pulling  the  cord  at  P,  the  weight,  W,  must 
ascend  as  much  and  as  fast  as  the  cord  is  drawn  down 
As,  therefore,  the  power  and  the  weight  move  with  the 
same  velocity,  it  is  clear  that  they  balance  one  another, 
and  that  no  mechanical  advantage  is  gained. 

In  all  the  applications  of  power  there  are  always  some 
directions  in  which  it  may  be  exerted  to  greater  advan- 
tage and  convenience  than  others;    and  in  many  cases 
the  power  is  capable  of  acting  in  only  one  particular  di- 
IL       rection.    Any  arrangement  of  machinery,  therefore,  which 
[vr\         Cfj    will  enable  us  to  render  power  more  available,  by  apply- 
ing it  in  the  most  advantageous  direction,  is  as  convenient 
and  valuable  as  one  which  enables  a  small  power  to  balance  or  overcome  a 


THE   ELEMENTS   OF    MACHINERY. 


103 


What  arc  fa- 
miliar applica- 
tions of  fixed 
pulleys? 


FlG.  82. 


great  weight.  Thus,  if  we  wish  to  apply  the  strength  of  a  horse  to  lift  a 
heavy  weight  to  the  top  of  a  building,  we  should  find  it  a  difficult  matter  to 
accomplish  directly,  since  the  horse  exerts  his 
strength  rnainty,  and  to  the  best  advantage,  in 
drawing  horizontally ;  but  by  changing  the  di- 
rection of  the  power  of  the  horse,  by  an  ar- 
rangement of  fixed  pulleys,  as  is  represented 
in  Fig.  81,  the  weight  is  lifted  most  readily, 
and  the  horse  exerts  his  power  to  the  best  ad- 
vantage. 

223.  A  fixed  pulley  is  most 
useful  for  changing  the  direc- 
tion of  power,  and  for  apply- 
ing    power     advantageously. 
By  it  a  man  standing  on  the  ground  can  raiso    '      l["    r  "  ' 

a  weight  to  the  top  of  a  building.  A  curtain,  a  flag,  or  a  sail,  can  be  readily 
raised  to  an  elevation  by  a  fixed  pulley,  without  ascending  with  it,  by  draw- 
ing down  a  cord  running  over  the  pulley. 

whatisamov-        224.  A   MOVABLE   PULLEY  differs  from   a 
able  pulley?      £xeci  pUney  in  being  attached  to  the  weight ; 
it  therefore  rises  and  falls  with  the  weight. 

Fig.  82  represents  a  movable  pulley,  B,  associated,  as  it 
most  common-ly  is,  with  a  fixed  pulley,  C.  The  movable  pulley, 
B,  is  often  called  a  "  Runner." 

225.  In  the  fixed  pulley,  Fig.  80,  it  will  be 
readily  seen  that  to  move  the  weight,  W,  at  f\ 
one  end  of  the  cord,  passing  over  the  pulley,  a 
greater  weight  must  be  applied  at  P,  for  if  P 
is  only  equal  to  W,  they  will  balance  one  an- 
other. If,  however,  we  fasten  one  end  of  the  cord  to  a  fixed  support,  as  at 
F,  Fig.  82,  and  pass  it  under  the  groove  in  the  movable  pulley  B,  to  which 
.the  weight,  W,  we  desire  to  raise  is  attached,  and  then  carry  it  over  the  fixed 
pulley  C,  we  may  lift  a  force  of  100  pounds  at  "W  by  an  application  of  50 
pounds  at  P.  To  understand  this,  we  must  remember  that  the  weight  "W"  is 
supported  by  the  cords  B  F  and  B  C  on  each  side  of  the  movable  pulley  B; 
and  as  each  are  equally  stretched,  the  weight  must  be  equally  divided  be- 
tween them ;  or,  in  other  words,  the  point  of  support,  F,  sustains  half  the 
weight,  and  the  power,  P,  the  other  half.  A  person,  therefore,  pulling  at  P, 
will  raise  the  weight  by  exerting  a  force  equal  to  its  half.  But  the  cord  at  P 
must  move  through  two  feet  to  raise  the  weight  "W"  one  foot. 

When  still  greater  power  is  required,  pulleys  are  compounded  into  a  system 
containing  two  more'  single  pulleys,  called  BLOCKS,  and  these  again  are  com- 
bined in  a  compound'  system  of  fixed  and  movable  pulleys. 

A  single  movable  pulley  may  be  so  arranged  that  the  power  will  sustain 
three  times  its  own  weight.  Such  an  arrangement  is  represented  in  Fig.  83. 


What  is  the  ad- 
vantage  gained 
by  the  use  of  a 
movable  pul- 

Icy? 


\V 


104 


WELLS'S   NATURAL   PHILOSOPHY. 


In  this  we  have  four  cords,  one  employed  in  sustaining  the  FlO.  83. 
power,  P,  and  the  other  sustaining  the  weight;  conse- 
quently the  power  will  be  to  the  weight  as  1  to  3.  In 
Fig.  84,  we  have  two  blocks;  each  containing  two  single 
pulleys.  The  rope  is  thus  divided  into  five  portions,  each 
equally  stretched;  one  is  employed  in  supporting  the 
power  P,  and  four  sustain  the  weight.  With  this  system 
a  power  of  1  wiU  balance  a  weight  of  4. 

How  is    ower          22Gt  Iu  ^  tliese  arrangements  of  pul- 

gained  at  the      leys,  the  increase  of  power  has  been  gamed 

thne'in6  a  sys-f     at  tlie  exPense   of  time,   and  the  space 

tem    of    pui-      passed  over  by  the  power  must  be  double 
the  space  passed  over  by  the  weight,  mul- 
tiplied by  the  number  of  pulleys.     That  is,  in  the  case  of 

the  single  pulley,  the  power  must  pass  over  two  feet  to 

raise  the  weight  one  foot;  and  with  two  movable  pulleys, 

as  in  Fig.  84,  the  power  must  fall  four  feet  to  raise  the 
FIG.  84.  weight  one  foot. 

Instead  of  folding  the  string  on  the  pulleys  entire,  it  is 
sometimes  doubled  into  separate  portions,  each  pulley 
hanging  by  a  separate  cord,  one  end  of  which  is  attached 
to  a  fixed  support.  Here  a  very  great  mechanical  advan- 
tage is  gained,  attended,  however,  with  a  corresponding 
loss  of  time.  In  an  arrangement  of  such  a  character,  re- 
presented in  Fig.  85,  the  weight  "W,  is  supported  by  the 
two  parts  of  the  cord  passing  round  the  movable  pulley, 
C ;  and  as  each  of  these  parts  is  equally  stretched,  the 
fixed  support  will  sustain  one  half  the  weight,  and  the 
next  pulley  in  order  above  C,  namely  B,  may  be  consid- 
ered as  sustaining  the  other  half.  But  the  two  parts  of 
the  string  which  support  the  pulley  B,  again  divide  the 
weight,  so  that  the  pulley  A,  which  is  attached  to  one  of 
them,  only  sustains  one  quarter  of  the  first  weight,  "W. 

The  string  which  passes  around  A  again  divides  this  ylG  35 

weight,  so  that  each  part  of  it  sustains  only  one  eighth 

of  W.     The  fixed  pulley  serves  merely  to  change  the 

direction  of  the  motion.     In  this  system,  therefore,  a 

power  of  1  will  balance  a  weight  of  8. 

227.  In  general,  the  advan- 

How  may  the  •       j   i_  Ti          •     /•         j 

'advantage         tage  gained  by  pulleys  is  found 

gained  by  pul-       T  ,,.,.,,  ,  /» 

leys  be  ascer-  by  multiplying  the  number  of 
movable  pulleys  by  two,  or  by 
multiplying  the  power  by  the  number  of 
folds  in  the  rope  which  sustains  the  weight, 
where  one  rope  runs  through  the  whole. 


THE    ELEMENTS   OF   MACHINERY. 


105 


Thus  a  weight  of  72  pounds  may  be  balanced  by  four  movable  pulleys  by 
a  weight  or  power  of  9  pounds ;  with  two  pulleys,  by  a  power  of  18  pounds, 
with  one  movable  pulley,  by  a  power  of  36  pounds. 

These  rules  apply  only  to  movable  pulleys  in  the  same  block,  when  the 
parts  of  the  rope  which  sustain  the  weight  are  parallel  to  each.  The  mechan- 
ical advantage  which  the  pulley  appears  to  possess  in  theory,  is  considerably 
diminished  in  practice,  owing  to  the  stiffness  of  the  ropes,  and  the  friction  of 
the  ropes  and  wheels.  From  these  causes  it  is  estimated  that  two  thirds  of 
the  power  is  lost.  "When  the  parts  of  the  cord  are  not  parallel,  the  strength 
of  the  pulley  is  very  greatly  diminished. 
Whatare  Cranes  J28.  Fixed  and  mov- 
and  Derricks,  able  pulleys  are  arranged 
Tackle  and  Fall?  in  ft  great  Varjet7  of 

forms,  but  the  principle  upon,  which  all  are 
constructed  is  the  same.  What  is  called  a 
."  tackle  and  fall,"  or  "  block  and  tackle," 
is  nothing  but  a  pulley.  Cranes  and 
derricks  are  pieces  of  mechanism  usually 
consisting  of  combinations  of  toothed 
wheels  and  pulleys,  by  means  of  which 
materials  are  lifted  to  different  elevations 
— as  goods  from  vessels  to  the  wharves, 
building  materials  from  the ;  ground  to 
the  stage  where  the  builders  are  en- 
gaged, and  for  similar  purposes.  One 
of  the  most  simple  forrijs  of  movable 
cranes  is  represented  in  Fig.  86.  It 
consists  of  a  strong  triangular  ladder,  at 
the  top  of  which  is  a  fixed  pulley,  C, 
over  which  the  rope  attached  to  the  ob- 
ject to  be  elevated  passes,  and  is  carried 
down  to  the  cylindrical  axle,  T,  upon 
which  it  is  wound  by  means  of  bars  in- 
serted in  holes,  or  by  a  crank.  This 
ladder  is  inclined  more  or  less  from  the 
upright  position  by  means  of  a  rope,  C 
D,  which  is  attached  to  some  fixed  point 
at  a  distance. 

229.  The  INCLINED  PLANE  consists  of  a  hard 
plane  surface,  inclined  at  an  angle. 

In  Fig.  87,  a  &  c  repre-  j»JG.  87. 

sents  an  inclined  plane.  c 

230.  If  we  attempt,  for 
instance,  to  raise  a  cask,  or  any  other 
'heavy  body  into  a  wagon,  we  may  find 
that  our  strength  is  unequal  to  lifting  it 

5* 


What  is  an  In- 
clined Plane? 


Illustrate  the 
use  of  an  In- 
clined Plane. 


106 


WELLS'S  NATURAL  PHILOSOPHY. 


How  do  we 
derive  a  me- 
chanical ad- 
vantage from 
an  inclined 
plane  ? 


HOW  can  we 


of 
plane  ? 


directly,  while  to  haul  it  up  by  pulleys  would  be  very  inconvenient,  if  not 
impossible.  We  may,  however,  accomplish  our  object  with  comparative  ease 
by  rolling  the  cask  up  an  inclined  plank,  and  exerting  our  force  in  a  direction 
parallel  to  the  inclined  surface  of  the  plank. 

The  plank,  in  this  instance,  forms  an  inclined  plane,  and  we 
gain  a  mechanical  advantage,  because  it  supports  a  part  of 
the  weight. 

If  we  place  a  body  upon  a  horizontal  plane,  or  surface,  it  is 
evident  that  the  surface  will  support  its  whole  weight  ;  if  we 
incline  the  surface  a  little,  it  will  support  less  of  the  weight,  and  as  we  elevate 
it  more,  it  will  continue  to  support  less  and  less,  until  the  surface  becomes 
perpendicular,  in  which  case  no  support  will  be  afforded. 

231.  The  advantage  gained  by  the  use  of  the  inclined  plane  may  be  esti- 
mated by  the  following  rule  : 

232.  The  power  is  to  the  weight  as  the  per- 
pendiculai  height  of  the  plane  is  to  its  length. 

Fr°m  this  {t  wil1  appear  that  the  leSS  the  hei&ht  °f  the  in* 
clined  plane,  and  the  greater  its  length,  the  greater  will  be 

the  mechanical  advantage.     Thus,  in  Fig.  88,  if  the  plane,  c 
d,  be  twice  as  long  as  the  height,  e  dt  FIG.  88. 

one  pound  at  p,  acting  over  the  pulley, 
would  balance  two  pounds  any  where 
between  c  and  d.  If  the  plane,  c  d, 
were  three  times  the  length  of  d  e, 
then  one  pound  at  p  would  balance 
three  pounds  any  where  on  the  plane, 
c  d,  and  so  of  all  other  quantities  and 
proportions. 

233.  Roads  which  are  not  level  may  be  considered  as  in- 
clined planes,  and  the  inclination  of  a  road  is  estimated  by 
the  height  which  corresponds  to  some  proposed  length.  Thus, 
we  say  a  road  rises  one  foot  in  twenty,  or  one  in  fifty,  mean- 
ing that  if  twenty  or  fifty  feet  of  the  road  be  taken,  as  the  length  of  an  in- 
clined plane,  the  corresponding  height  of  such  a  plane  would  be  one  foot,  and 
the  difference  of  level  between  the  two  extremities  of  such  a  length  of  road 
would  be  one  foot. 

According  to  this  method  of  estimating  the  inclination  of 
roads  to  be  roads,  the  power  required  to  sustain,  or  draw  up  a  load,  fric- 
constructed  ?  tion  not  considered,  is  always  proportioned  to  the  rate  of  ele- 
vation. On  a  level  road,  the  carriage  moves  when  the  horse  exerts  a  strength 
sufficient  to  overcome  the  friction  and  resistance  of  the  atmosphere  ;  but  in 
going  up  a  hill,  where  the  road  rises  one  foot  in  twenty,  the  horse,  beside 
these  impediments,  is  obliged  to  exert  an  extra  force  in  the  proportion  of  one 
to  twenty,  or,  in  other  words,  he  is  obliged  to  lift  one  twentieth  of  the  load. 
It  is,  therefore,  bad  policy  ever  to  construct  a  road  directly  over  the  summit 
of  a  hill,  when  it  can  be  avoided,  because,  in  addition  to  the  force  necessary 


How  do  we  es- 
timate the  in- 
clination of 
roads  ? 


THE   ELEMENTS   OF   MACHINERY.  107 

to  overcome  the  friction  in  drawing  a  heavy  load  up  the  steep  incline,  we 
must  add  additional  force  to  overcome  the  gravity,  which  acts  parallel  with 
the  inclined  plane  of  the  road,  and  tends  constantly  to  make  the  load  roll 
back  to  the  bottom  of  the  slope.  This  force  increases  most  rapidly  with  the 
steepness,  and  consequently  requires  an  immense  expenditure  of  power. 
An  equal  power  expended  on  a  road  gently  winding  round  the  hill,  with  an 
increase  of  speed,  would  gain  the  same  elevation  in  much  less  time. 

An  intelligent  driver,  in  ascending  a  steep  hill  on  which  there  is  a  broad 
road,  winds  from  side  to  side,  since  by  so  doing  he  diminishes  the  abruptness 
of  the  ascent  (the  plane  being  made  longer  in  proportion  to  its  height),  and 
thus  favors  the  horses. 

Our  common  stairs  are  inclined  planes,  the  steps  being  merely  for  the  pur- 
pose of  obtaining  a  good  foot-hold. 

234.  In  the  inclined  plane,  as  in  all  other  simple  machines, 
gained8  at°The  a  £a'n  m  Power  *s  attended  with  a  corresponding  loss  of  time, 
expense  oftime  A  body,  in  ascending  an  inclined  plane,  lias  a  greater  space 
pianeVQC  to  pass  over  than  if  it  should  rise  perpendicularly.  The  time, 

therefore,  of  its  ascent  will  be  greater,  and  it  will  thus  oppose 
less  resistance,  and  consequently  require  less  power. 

what  is  a  235.  The  WEDGE  is  a  movable       FIG.  89. 

wedge?         inclined  plane.     It  is  also  defined    ^          B 
to  be  two  inclined  planes  united  at  their  bases, 
as  A  B,  Fig.  89. 

In  the  inclined  plane,  the  weight  moves  upon  the  plane, 
which  remains  stationary ;  but  in  the  wedge,  the  plane  itself 
is  moved  under  the  weight. 

236.  The  cases  in  which  wedges  are  most 
are  Wedges  generally  used  in  the  arts,  are  those  in  which 
art*?  in  tbe  an  intense  force  is  required  to  be  exerted  through  a  very  small 
space.  It  is,  therefore,  used  for  splitting  masses  of  wood,  or 
stone,  for  blocking  up  buildings,  raising  vessels  in  docks,  and  pressing  out  the 
oil  from  seeds.  In  this  last  instance,  the  seeds  are  placed  in  bags,  between 
two  surfaces  of  hard  wood,  which  are  pressed  together  by  wedges. 

Upon  what  237.  The  usefulness  of  the  wedge  depends 

ence8thoVr1h;  on  friction  ;  for  if  there  were  no  friction,  the 
plndf6  de~  wedge  would  fly  back  after  each  stroke  of  the 

driving  force. 

HOW  does  the  238.  The  power  of  the  wedge  increases  as 
wTdegefa-the  the  length  of  its  back,  compared  with  that  of 

its  sides,  is  diminished.   Hence,  it  follows  that 
the  power  of  the  wedge  is  in  proportion  to  its  sharpness. 

The  power  commonly  used  in  the  case  of  the  wedge,  is  not  pressure,  but 
percussion.'  Ita  edge  being  inserted  into  a  fissure,,  the  wedge  is  driven  in  by 


108 


WELLS'S  NATURAL  PHILOSOPHY. 


What  are  fa- 
miliar exam- 
ples of  the  use 
or  application 
of  the  Wedge 
in  the  arts  ? 


FIG.  90. 


What  is  the 
Thread  of  a 
Screw  1 


blows  upon  its  back.  The  tremor  produced  when  the  wedge  is  struck  with 
a  violent  blow,  causes  it  to  insinuate  itself  much  more  rapidly  than  it  other- 
wise would. 

239.  The  edges  of  all  cutting  and  piercing  instruments, 

such  as  knives,  razors,  chisels,  nails,  pins,  etc.,  are  wedges. 

The    angle    of   the   wedge  in  all  these  cases  is  more  or 

less  acute,  according  to  the  purpose  to  which  it  is  applied. 

Chisels  intended  to  cut  wood  have  their  edges  at  an  angle  of 
about  30°  ;  for  cutting  iron  from  50°  to  60°,  and  for  brass  about  80°  to  90°. 
In  general,  tools  which  are  urged  by  pressure  admit  of  being  sharper  than 
those  which  are  driven  by  percussion.  The  softer,  or  more  yielding  the  sub- 
stance to  be  divided  is,  the  more  acute  the  wedge  may  be  constructed. 

what  is  the          240.  The  SCREW  is  an  inclined  plane  wind- 
screw?          jng  roun(|  a  cylinder. 

This  may  be  illustrated  by  cutting  a  strip 
of  paper  in  such  a  way  as  to  represent  an  in- 
clined plane,  and  then  winding  it  round  a 
cylinder,  or  common  lead  pencil,  as  is  repre- 
sented in  Fig.  90. 

241.  The  edge  of  the 
inclined  plane  winding 
about  the  cylinder,  or 

the  coil  of  the  spiral  line  which 
it  describes  upon  the  cylinder,  con- 
stitutes the  THREAD  of  the  screw, 
and  the  distance  between  the  suc- 
cessive coils  is  called  the  DISTANCE 

BETWEEN  THE  THREADS. 

The  screw,  surrounded  by  its  spiral  line  is  represented  in  Fig.  91. 

The  screw  is  not  applied  directly  to  the  resistance  to  be        FIG.  91. 
overcome,  as  in  the  case  of  the  inclined  plane  and  wedge,  but 
the  power  is  transmitted  by  means  of  what  is  called  the  NUT. 

242.  The  NUT  of  a  screw  is  a 
block)   ^fa  &   cylindrical   cavity, 

having  a  spiral  groove  cut  round  upon  the 
surface  of  this  cavity  corresponding  with  the 
thread  of  the  screw. 

In  this  groove  the  thread  of  the  screw  will  move  by  causing 
the  screw  to  rotate.     Each  turn  of  the  screw  in  the  nut  will  cause  it  to  advance 
or  recede  a  distance  just  equal  to  the  interval  between  the  threads. 
Is  the  Screw,         Generally,  the  nut  is  stationary  and  the  screw  movable,  but 
*^  wt  mav  be  ^PY*^  *&&  the  acrew  stationary. 


what  is  the 


THE  ELEMENTS   OF   MACHINERY. 


•109 


How  is  power 
applied  to  the 
Screw  ? 


FIG.  92. 


243.  Power  is  commonly  applied  to  the  screw  by  means  of 
a  lever,  either  attached  to  the  nut,  or  to  the  head  of  the  screw, 
as  seen  in  Fig.  92.     By  varying  the  length  of  this,  the  power 

may  be  indefinitely  increased  at  the  point  of  resistance.     The  screw,  there- 
fore, acts  with  the  combined  power  of  the  lever  and  the  inclined  plane. 

Thus,  in  Fig.  92,  fd  is  the  lever,  c  the  nut, 
'a  d  the  screw,  and  e  the  block  upon  which  the 
substance  to  be  pressed  is  placed.  As  in  all  the 
other  simple  machines,  the  advantage  in  this  is 
Estimated  by  the  relative  distances  passed  over 
by  the  power  and  the  weight.  If  the  distance 
of  the  spiral  threads  of  the  screw  is  1  inch,  and 
the  handle  of  the  screw,  that  is  the  lever,  is  2 
feet  in  length,  then  the  extremity  of  the  lever 
will  describe  a  circle  of  over  12  feet  in  turning 
once  round,  but  the  screw  will  only  advance  1 
inch.  The  ratio  between  the  power  and  the 
--  -  •  —  weight  will  be,  therefore,  as  1  inch  to  12  feet,  or 
as  1  to  144.  Consequently,  if  a  man  is  capable 

of  exerting  a  force  of  60  pounds  at  the  end  of  the  lever,  the  screw  will  ad- 
vance with  a  force  of  8,640  pounds.  If  the  distance  of  the  threads  had  been 
4  an  inch,  the  power  exerted  by  the  screw  would  have  been'  doubled.  In 
this  illustration  friction  has  not  been  taken  into  account  ;  this  will  diminish 
the  total  effect  nearly  one  fourth. 

244.  The  advantage  gained  by  the  screw  is 
th     ce           proportion  as  the  circumference  of  the  circle 

estimated?  described  by  the  power  (that  is  by  the  handle 
of  the  lever)  exceeds  the  distance  between  the  threads  of 
the  screw. 

Hence  the  enormous  mechanical  force  exerted  by  the  screw  is  rendered 
evident.  There  is  no  limit  to  the  smallness  of  the  distance  between  the 
threads  except  the  strength  it  is  necessary  to  give  them  ;  and  there  is  no  limit 
to  the  magnitude  of  the  circumference  to  be  described  by  the  power,  except 
the  necessary  facility  for  moving  it.  FIG.  93. 

What  are  fa-  245'     The    SC1<eW    is 

miiiar  applies-      generally  used  where 

ScrTw?*  th6  ^eat  Pressure  ^  t°  ke 
exerted  through  small 
spaces  ;  hence  its  application  in  presses 
of  all  kinds;  for  extracting  the  juices 
of  seeds  and  fruits,  in  compressing  cot- 
ton, hay,  etc.,  as  also  for  coining  and 
punching.  For  the  two  latter  opera- 
tions it  is  -caused  to  act  with  enor- 


HOW  is  the  ad- 


110 


WELLS'S   NATURAL  PHILOSOPHY. 


What  IB  an 
endless  Screw  ? 


Describe     the 
construction 
and  advantage 
of      Hunter's 
Screw. 


FIG.  94. 


mous  energy  by  means  of  the  momentum  of  two  heavy  balls  attached  to  the 
end  of  a  long  lever,  or  handle,  as  is  represented  in  Fig.  93.  A  force  of  sev- 
eral tons  may  thus  be  applied  at  one  effort. 

When  the  thread  of  a  screw 

works  hi  the  teeth  of  a  wheel,  as 

is  shown  in  Fig.  94,  it  constitutes 
what  is  called  an  endless  screw.  Such  a  con- 
trivance is  oftentimes  a  very  convenient  method 
of  applying  power. 

246.  The  efficacy  of  a  screw 

increases  with  the  fineness  of 

the  thread ;  but  a  practical  limit 

is  soon  attained,  for  if  the  thread 

be  made  too  fine,  it  will  become 
weak,  and  be  liable  to  be  torn  off. 


FIG.  95. 


To  obtain 

an  indefinite  increase  of  the  strength  of  the  screw 
without  diminishing  the  strength  of  the  thread,  we 
have  a  contrivance  known  as  "  Hunter's  screw,"  rep- 
resented  in  Fig.  95.  It  consists  of  a  screw,  A,  work- 
ing in  a  nut.  To  a  movable  bottom-board,  D,  a  sec- 
ond screw,  B,  is  affixed.  This  second  screw  works  in 
the  interior  of  A,  which  is  hollow,  and  in  which  a 
corresponding  thread  is  cut.  When,  therefore,  A  is 
screwed  downward,  the  threads  of  B  pass  upward,  and 
the  movable  piece,  D,  urged  forward  by  the  screw 
which  has  the  greater  thread,  it  is  drawn  back  by  that 
which  has  the  less ;  so  that  during  each  revolution  the 
'  screw  instead  of  being  advanced  through  a  space  equal 

to  the  breadth  of  either  of  the  threads,  moves  through  a  space  equal  to  their 
difference.  Suppose  the  distance  between  the  threads  of  A  to  be  l-20th  of 
an  inch,  and  of  B  1-2 1st  of  an  inch ;  then  in  turning  the  screw  A  once,  the 
board  D  will  descend  a  distance  equal  to  the  difference  between  l-20th  and 
l-21st,  or  the  l-420th  of  an  inch.  Hence,  if  the  circle  described  by  the  han- 
dle be  26  inches  while  the  screw  advances  l-420th  of  an  inch,  the  power  will 
be  to  the  weight  as  1  to  8,400. 

247.  All  machines,  however  complicated,  are  made  up  of  combinations  of 
the  six  simple  machines.  If  we  examine  the  construction  of  any  complex  ma- 
chine, as  a  steam-engine,  a  loom,  a  spinning  machine,  or  a  time-piece,  we 
shall  find  that  they  are  composed  of  simple  levers,  wheels  and  axles, 
screws,  etc.,  connected  together  in  an  endless  variety  of  forms,  to  form  a 
complete  whole. 

In  the  practical  application  of  machinery,  it  rarely  or  never 
force   in  ma-      happens  that  the  moving  force  is  capable  of  producing  directly, 
^/^tT     tfP?     *^ie  Par^cular  kind  of  motion  required  by  the  machine  to  per- 
form the  work  to  which  it  is  adapted.     Expedients  must 
therefore  be  resorted  to,  by  means  of  which  the  motions  which  the  moving 


,THE    ELEMENTS   OF    MACHINERY.  Ill 

power  is  capable  of  exerting  directly  can  be  converted  into  those  which  are 
necessary  for  the  purposes  to  which  the  machine  is  applied. 

HOW  many  248.  The  varieties  of  motion  which  occur  in 
«0nndsar°ef  c™£  machinery  are  divided  into  two  classes,  viz. : 

cMnery?inma~      ROTARY  and  RECTILINEAR  MOTION. 

what  is  Rota-        249.  In  Rotary   Motion,  the  several  parts 
revolve  round  an  axis,  each  performing  a  com- 
plete circle,  or  similar  parts  of  a  circle,  in  the  same  time, 
what  ia  Rec         ^^  *n  Rect^mear  Motion,  the  several  parts 
MO-    of  a  moving  body  proceed  in  parallel  straight 
lines  with  the  same  speed. 

Examples  of  rotary  motion  are  seen  in  all  kinds  of  wheel  work,  and  exam- 
ples of  rectilinear  motion  in  the  rod  of  a  common  pump,  the  piston  of  a  steam- 
engine,  the  motion  of-  a  straight  saw. 

WhatisReci  ^n  ro^ar7  an^  rectilinear  motion,  if  the  parts  move  con- 

rocating  Mo-  stantly  in  the  same  direction,  the  motion  is  called  continued 
rotary,  or  continued  rectilinear  motion.  If  the  parts  move 
alternately  backward  and  forward  in  opposite  directions,  passing  over  the 
same  spaces  from  end  to  end  continually,  the  motion  is  called  reciprocating 
motion. 

How  are  rota-  251-  Tne  method  by  which  a  power  having  one  of  these 
ryand  rocipro-  motions  may  be  made  to  communicate  the  same  or  a  different 
catory  motion  ,.-,/..  .,  ,  ,  ,  ..  f 

converted  into      kind  of  motion,   involves  a   lengthy  description  of  a  great 

each  other  ?  variety  of  machinery ;  but  the  most  simple  and  common  plan 
of  converting  rotary  motion  into  rectilinear,  and  rectilinear  motion  back  again 
into  rotary,  is  by  means  of  what  is  called  a  Crank. 

what   is    a        252.  The  CRANK  is  a  double  winch,  or  han- 
crank?        ^  an(j  jg  fomiec[  by  bending  an  axle  so  as 
to  form  four  right  angles,  facing  in  opposite  directions. 

It  is  represented  complete  in  Fig.  96.     Attached  q 

to  the  middle  of  C  D,  by  a  joint,  G,  is  a  rod,  H, 
which  is  the  means  of  imparting  power  to  the  crank. 
This  rod  is  driven  by  an  alternate  motion,  like  the 
brake  of  a  pump.  The  bar  C  D  is  turned  with  a 
circular  motion  round  the  axle  A  F.* 
What  disad-  ^e  disadvantage  attending  the 
vantages  at-  use  of  the  crank  is,  that  it  is  incapa- 
oTthe  crank  I*6  ^le  °^  transmitting  a  constant  force 

to  the  resistance.     This  is  illustrated  in  Fig.  97.     In  No.  1, 

*  The  terms  axis,  axle,  arbor,  and  shaft,  in  mechanics,  are  generally  understood  to 
mean  the  bar,  or  rod,  which  passes  through  the  center  of  a  wheel.  A  gudgeon  is  the  pin, 
or  support,  on  which  a  horizontal  shaft  turns ;  the  pins  upon  which  an  upright  shaft  turni 
are  called  pivots. 


112 


WELLS'S   NATURAL  PHILOSOPHY. 


No.  1. 


No.  2. 


what 


FIG.  9T.  where  the  arm  of  the  crank  is  horizontal,  the  power 

from  the  rod  acts  with  the  greatest  advantage,  as 
at  the  extremity  of  a  lever.  But  when  the  rod 
which  communicates  motion  stands  perpendicular 
with  the  arm  of  the  crank,  as  in  No.  2,  which  is. 
the  case  twice  during  every  revolution,  the  power, 
however  great,  can  exert  no  effect  upon  the  resist- 
ance, the  whole  force  being  expended  in  producing 
pressure  upon  the  axle  and  pivots  of  the  crank. 
Such  a  situation  of  the  rod  and  the  arm  of  the 
crank  is  called  the  dead  point,  and  when  the  ma- 
hinery  stops,  as  is  often  the  case,  it  is  said  to  be 
"  set,"  or  "caught  on  its  center."  The  difficulty  is 
generally  overcome  by  the  employment  of  a  fly. 
wheel  (§  21),  which,  by  its  inertia,  keeps  up  the 
motion. 

SECTION    II. 

FRICTION. 

proper-  253.  The  most  serious  obstacle  to  the  per- 
fection of  machinery  is  Friction  ;  and  it  is 
lost  by  friction?  usually  considered  to  destroy  one  third  of  the 
power  of  a  machine. 

254.  Friction  is  of  two  kinds  :  sliding  and 
kinds  of  fric-    rolling.     Sliding  friction  is  produced   by  the 

tion  are  there?  K  „  *  J . 

sliding,  or  dragging  of  one  surface  over  another ; 
rolling  friction  is  caused  by  the  rolling  of  a  circular  body 
upon  the  surface  of  another. 

Friction  increases  as  the  weight,  or  pressure  increases,  as 
the  surfaces  in  contact  are  more  extensive,  and  as  the  rough- 
ness of  the  surfaces  increase.      With  surfaces  of  the  same 
material,  friction  is  nearly  proportional  to  the  pressure. 

Friction  diminishes  as  the  weight  or  pressure  is  less,  as  the 
tion  diminish  ?  polish  or  smoothness  of  the  moving  surfaces  is  more  perfect, 
and  as  the  surfaces  in  contact  are  smaller.  It  may  also  be 
diminished  by  applying  to  the  surfaces  some  unguent,  or  greasy  material: 
oils,  tallow,  black-lead,  etc.,  are  commonly  used  for  this  purpose ;  they  dimin- 
ish friction  by  filling  up  the  minute  cavities  and  smoothing  the  irregularities 
that  exist  upon  the  surface.*  Oils  are  the  best  adapted  for  diminishing  the 
friction  of  metals,  and  tallow  the  friction  of  wood. 

*  All  bodies,  however  .much  they  may  be  polished,  appear  rough  and  uneren  when 
examined  with  a  microscope. 


FRICTION.  113 

t    r    th  ^^'  Friction,  although  an  obstacle  in  the  working  of  ma- 

advantages  of  chinery  generally,  is  not  without  some  advantages.  Without 
friction1/  friction,  the  stones  and  bricks  used  in  building  would  tend  to 

fall  apart  from  one  another.  When  nails  and  screws  are  driven  into  bodies, 
with  a  view  of  holding  them  together,  it  is  friction  alone  that  maintains  them 
in  their  places.  The  strength  of  cordage  depends  on  the  friction  of  the  short 
libers  of  the  cotton,  flax,  or  hemp,  of  which  it  is  composed,  which  prevents 
them  from  untwisting.  In  walking,  we  are  dependent  on  friction  for  our 
foothold  upon  the  ground:  the  difficulty  of  walking  upon  smooth  ice  illus- 
trates this  most  clearly.  Without  friction  we  could  not  hold  any  body  in  the 
hand;  the  difficulty  of  holding  a  lump  of  ice  is  an  example  of  this.  Without 
friction,  the  locomotive  could  not  propel  its  load ;  for  if  the  tire  of  the  driving 
wheel  and  the  rail  were  both  perfectly  smooth,  one  would  slip  upon  the  other 
without  affording  the  requisite  adhesion. 

256.  Experiments  seem  to  show  that  the  friction  of  two 
tion  between  surfaces  of  the  same  substance  is  generally  greater  than  the 
different  sub*  friction  of  two  unnko  substances.  The  friction  of  polished 
stances  com-  steel  against  polished  steel,  is  greater  than  that  of  polished 
Pare?  steel  upon  copper,  or  on  brass.  So  of  wood  and  various 

other  metals. 

25*7.  For  transporting  very  heavy  timbers,  or  large  castings, 
d  wheels  of  great  size  are  used,  as  by  their  use  the  weight  is 
for  transport-  moved  with  greater  facility,  and  the  roughness  of  the  road 
wfights?  eaVy  more  easily  overcome  than  with  small  wheels.  The  reason 
of  this  is,  that  the  large  wheels  bridge  over  the  cavities  of  tho 
road,  instead  of  sinking  into  them ;  and  in  surmounting  an  obstacle,  the  large 
circumference  of  the  wheel,  causes  the  load  to  rise  very  gradually. 

The  resistance  of  sliding  friction  is  much  greater  than  that  of  rolling  fric- 
tion. In  the  wheel  of  a  carriage  there  is  rolling  friction  at  the  circumference 
of  the  wheel,  but  sliding  friction  at  the  axles.  In  a  locomotive,  the  so-called 
driving  wheels  are  turned  by  the  force  of  the  steam-engine ;  the  whole  car- 
riage rolls  on  in  consequence  of  this  rotation ;  for  if  the  locomotive  were  to 
remain  at  rest,  the  wheels  could  not  revolve  without  sliding  on  the  rails,  and 
overcoming  a  great  amount  of  sliding  friction ;  but  by  rolling,  the  wheels  have 
only  the  much  smaller  rolling  friction  to  overcome.  The  machine,  therefore, 
moves  onward,  this  being  the  direction  in  which  its  motion  will  experience 
the  least  resistance. 

The  load  which  a  locomotive  is  capable  of  drawing  depends,  not  only  upon 
the  force  of  its  steam  power,  but  also  upon  the  weight  of  the  engine,  or,  in 
other  words,  upon  the  pressure  of  the  driving  wheels  upon  the  rails,  the  fric- 
tion increasing  with  the  pressure.  If  we  assume  that  two  locomotives  have 
equally  strong  engines,  but  that  one  is  heavier  than  the  other,  a  greater 
weight  will  be  propelled  by  the  heavier  of  the  two. 

Friction  is  generally  resorted  to  as  the  most  convenient  method  of  retard- 
ing the  motion  of  bodies,  and  bringing  them  to  rest.  The  different  modifica- 
tions of  machinery  employed  for  this  purpose  are  termed  Brakes. 


114  WELLS'S  NATURAL  PHILOSOPHY. 


PRACTICAL   PROBLEMS   IN   MECHANICS. 

1.  What  must  be  the  horse-power  of  a  locomotive  engine  which  moves  at  the  constant 
speed  of  '25  miles  per  hour,  on  a  level  track,  the  weight  of  the  train  being  60  tons,  and  the 
resistance  from  friction  being  equal  to  4SO  pounds  ? 

2.  If  a  lever,  twelve  feet  long,  have  its  fulcrum  4  feet  from  the  weight  at  one  end,  and 
this  weight  be  12  pounds,  what  power  at  the  other  end  will  balance  ? 

3.  In  a  lever  of  the  first  class  a  power  of  20  at  one  end  balances  a  weight  of  100  at  tha 
other:  what  is  the  comparative  length  of  the  two  arms? 

4.  In  a  lever  of  the  first  class,  6  feet  in  length,  the  power  is  75,  and  the  weight  150 
pounds:  where  must  the  fulcrum  be  placed  in  order  that  the  two  may  balance? 

5.  Two  persons  carry  a  weight  of  200  pounds  suspended  from  a  pole  10  feet  long ;  one 
of  them  being  weak  can  carry  only  75  pounds,  leaving  the  rest  of  the  load  to  be  carried 
by  the  other :  how  far  from  the  end  of  the  pole  must  the  weight  be  suspended  ? 

6-  A  lever  of  the  second  clans  is  20  feet  long :  at  what  distance  from  the  fulcrum  must 
a  weight  of  SO  pounds  be  placed  in  order  that  it  may  be  sustained  by  a  power  of  60 
pounds  ? 

7  In  a  lever  of  the  third  class,  8  feet  long,  what  power  will  be  required  to  balance  a 
weight  of  100  pounds,  the  power  being  applied  at  a  distance  of  2  feet  from  the  fulcrum  ? 

8 .  A  power  of  5  pounds  is  required  to  lift  a  weight  of  20,  by  means  of  the  wheel  and 
axle :  what  must  be  the  proportionate  diameters  of  the  wheel  and  axle  ? 

9.  A  power  of  60  acts  on  a  wheel  8  feet  in  diameter :   what  weight  suspended  from  a 
rope  winding  round  an  axle  10  inches  in  diameter  will  balance  this  power  ? 

10  In  a  set  of  cog-wheels  the  diameters  of  wheel  and  axle  are,  first  7  and  2, 
second  8  and  1,  third  9  and  1  :  a  power  of  25  being  applied  at  the  circumference  of  the 
first  wheel,  what  weight  will  be  sustained  at  the  axle  of  the  third  ? 

11.  What  weight  will  a  power  ol  3  sustain  with  a  system  of  4  movable  pulleys,  one 
cord  passing  round  all  of  them  ? 

12.  Suppose  a  power  of  100  pounds  applied  to  a  set  of  2  movable  pulleys,  what  weight 
will  it  sustain,  allowing  a  deduction  of  two  thirds  for  friction  ? 

13.  If  a  man  is  able  to  draw  a  weight  of  200  pounds  up  a  perpendicular  wall  10  feet 
high,  how  much  will  he  be  able  to  draw  up  a  plank  40  feet  long,  sloping  from  the  top  of 
the  wall  to  the  ground,  no  allowance  beiiig  made  for  friction  ? 

Solution.—  In  this  the  height  (10)  is  to  the  length  (40)  as  the  weight  (200)  is  to  the  re- 
quired weight. 

14.  If  a  man  has  just  strength  enough  to  lift  a  cask  weighing  1%  pounds  perpendicu- 
larly into  a  wagon  3  feet  high,  what  weight  could  he  raise  by  means  of  a  plank  10  feet 
long,  with  one  end  resting  upon  the  wagon,  and  the  other  on  the  ground  ? 

15.  The  length  of  a  plane  is  12  feet,  the  height  is  4  feet :  what  is  the  proportion  of  the 
power  to  the  weight  to  be  raised  ? 

16  The  distance  between  the  threads  of  a  screw  being  half  an  inch,  and  the  circumfer- 
ence describad  by  the  power  10  feet,  what  proportion  will  exist  between  the  power  and 
the  weight  ? 

Solution.— The  power  will  be  to  the  weight  as  half  an  inch,  the  distance  between  the 
threads,  is  to  10  feet  (240  half  inches),  the  circumference  described  by  the  power— 1  to  240. 

17.  A  power  of  20  pounds  acting  at  the  end  of  a  lever  attached  to  a  screw  describes  a 
circle  of  100  inches:  whnt  resistance  will  the  power  overcome,  the  distance  between  tho 
threads  of  the  screw  being  2  inches? 


If;  :.:.^v"  I.    . 


CHAPTER    VII. 

ON  THE  STRENGTH  OF  MATERIALS  USED  IN  THE  ARTS,  AND 
THEIR  APPLICATION  TO  ARCHITECTURAL  PURPOSES. 


SECTION    I. 

ON  THE  STRENGTH  OP  MATERIALS. 

Upon  what  does  258.  WHEN  materials  are  employed  for 
mechanical  purposes,  their  power,  or  strength, 
£or  resisting  external  force,  apart  from  the  na- 
ture of  the  material,  depends  upon  the  shape  of  the 
material,  its  bearing,  or  manner  of  support,  and  the  nature 
of  the  force  applied  to  it. 

^59.  A  beam,  or  bar,  will  sustain  the  greatest 
application  of  force,  when  the  strain  is  in  the 
£rce  ?reatest      direction  of  its  length. 

260.  The  strongest  of  all  metals  for  resisting  tension,  or  a 
strength  of  dtf-  cforect  pull,  is  iron  in  the  condition  of  tempered  steel.  The 
ferent  substan-  strength  of  metals  is  affected  by  their  temperature,  being 
diminished,  in  general,  as  their  temperature  is  raised.  Wood 
of  the  same  kind  is  subjected  to  very  great  variations  of  strength.  Trees 
that  grow  in  mountainous  or  windy  places,  have  greater  strength  than 
those  which  grow  on  plains ;  and  the  different  parts  of  a  tree,  such  as  the 
root,  trunk,  and  branches,  possess  different  degrees  of  strength.  Cords  of 
equal  thickness  are  strong  in  proportion  to  the  fineness  of  their  strands,  and 
also  to  the  fineness  of  the  fibers  of  these  strands.  Ropes  which  are  damp, 
are  stronger  than  those  which  are  dry;  those  which  are  tarred  than  the  un- 
tarred,  the  twisted  than  the  spun,  the  unbleached  than  the  bleached.  Other 
things  being  equal,  a  rope  of  silk  is  three  times  stronger  than  a  rope  of  flax. 

HOW  does  the  261.  Of  two  bodies  of  similar  shape,  but  of 
affeectf  ftsbody  different  sizes,  the  larger  is  proportionably  the 
strength?  weaker.* 

*  A  knowledge  of  the  strength  of  various  materials  in  resisting  the  action  of  forces  ex- 
erted in  different  directions,  is  of  great  importance  in  the  arts.  In  the  following  tables 
are  collected  the  results  of  the  most  recent  and  extensive  experiments  upon  this  subject 
The  bodies  subjected  to  experiment  are  supposed  to  be  in  the  form  of  long  rods,  the  cross- 


116 


WELLS'S  NATURAL  PHILOSOPHY. 


in  what  posi- 


That  a  large  body  may  have  the  proportionate  strength  of  a  smaller,  it  must 
contain  a  greater  proportionate  amount  of  material  ;  and  beyond  a  certain 
limit,  no  proportions  whatever  will  keep  it  together,  but  it  will  fall  to  pieces 
by  its  own  weight.  This  fact  limits  the  size,  and  modifies  the  shape  of  most 
productions  of  nature  and  art—  of  trees,  of  animals,  of  architectural  or  mechan- 
ical structures. 

262.  The  strength  of  a  rectangular  beam,  or 
a  beam  in  the  form  of  a  parallelogram,  when 
the  strongest?  ^g  narrow  side  is  horizontal,  is  greater  than 
when  its  broad  side  is  horizontal,  in  the  same  proportion 
that  the  width  of  its  broad  side  is  greater  than  the  width 
of  its  narrow  side. 

Hence,  in  all  parts  of  structures  where  beams  are  subjected  to  transverse 
strain,  as  in  the  rafters  of  roofs,  floors,  etc.,  they  are  always  placed  with  their  * 
narrow  sides  horizontal,  and  their  broad  sides  vertical. 

section  of  which  measures  a  square  inch  ;  in  the  second  column  is  given  the  amount  of 
breaking  weights,  which  are  the  measure  of  their  strength  in  resisting  a  direct  pull. 


Name. 
1st.  Metals;— 
Steel,  tempered  from 
Iron,  bar  " 
—     plate,  rolled.       " 
—     wire            ..       " 

Ibs.              Ibs. 

114T94  to  153471 
53182  —  84611 
53920 
58736  —112905 

72064 
55S72 
16243  —  19464 
40997 
20320  —  37380 
37770  —  39968 
17047  —  19472 
47114—  58931 
52240 
20490  —  65237 
3228—    6666 

Name. 
Metals  ;— 
Tin,  cast. 

.  .  .  from 

Zinc.  . 

Lead,  wire  

U 

2d.  Woods;— 
Teak  

—      Swedish  mal 
leable  ....       " 
—     English    do.       " 
—      cast  " 
Silver,  cast        " 

Sycamore  
Beech  

...          " 

Elm  

(1 

Larch 

it 

—        hammered.     " 
Brass  cast                     " 

Oak.,  

U 

Alder 

44 

Box 

44 

—      plate  " 
Gold  " 

Ash 

44 

Pine  

M 

Tin.  .  . 

Fir... 

W 

4736 
1820 
2543  to 


S823 


12915—15405 
9630 
12225 

9720  —  15040 
10240 

10367  —  25851 
11453  —  21730 
14210  —  24043 
134SO  —  23455 
10038  —  14965 
6991  —  12876 

The  following  table  shows  the  average  weights  sustained  by  wires  of  different  metals, 
each  having  a  diameter  of  about  one  twelfth  of  an  inch  ; 

Lead 27  pounds. 

Tin 34 

Zinc 109       " 

Gold 150        " 


Silver 187  pounds. 

Platinum 274       " 

Copper 303        " 

Iron...  .  549 


Cords  of  different  materials,  but  of  the  same  diameter,  sustained  the  following  weights : 

Common  flax 1175  pounds.      New  Zealand  flax 2380  pounds. 

Hemp 1633       » 


Silk 3400 


Ibs. 


IbB. 


The  following  table  shows  the  weights  necessary  to  crush  columns  or  pillars  composed 
of  different  metals ;  the  numbers  expressed  in  the  second  column  being  the  total  crush- 
ing weight  in  Ibs.  per  square  inch  : 

Name.  Ibs.         lb». 

2d.  Woods:— 

Oak from  3860  to  514T 

Pine ...  "  .  1928 

Elm "    1284 

3d.  Stones:— 

Granite "    4970 

Sandstone "     2556 


Name. 

1st  Metals: 
Cast  iron from  1 1 581 3  to  177776 


Brass,  fine 

Copper,  molten 

—        hammered. 

Tin,  molten 

molten 


164864 

117088 

103040 

15456 

7728 


Brick,  well  baked. 


1092 


ON   THE   STRENGTH   OF   MATERIALS. 


117 


The  strength  of  a  structure  depends,  in  a  very  great  degree,  on  the  manner 
in  which  the  several  parts  are  joined  together,  and  by  a  skillful  combination, 
or  interlocking,  very  weak  and  fragile  materials  may  be  made  to  resist  the 
action  of  powerful  forces.  Examples  of  this  occur  in  the  manufacture  of 
ropes,  strings,  thread,  etc.  ;  in  the  weaving  of  baskets,  and  especially  in  the 
structure  of  cloth  ;  in  this  last  instance,  a  series  of  parallel  threads  called  the 
-p  „  QQ  woof,  is  made  to  interlock  with 

another  series  of  threads  called 
the  warP>  running  transversely 
across,  and  passing  alternately, 
over  and  under  the  first  series. 
Fig.  98  represents  the  appear- 
ance of  a  piece  of  plain  cloth 
seen  through  the  'microscope; 
the  alternate  intersections  of 
the  threads  are  seen  in  the 


threads,  and  the  cross  line  the  woof. 

263.  When  a  single  beam  can  not  be  found  deep  enough 
to  have  the  strength  required  in  any  particular  case,  several 
beams  may  be  joined  together,  in  a  variety  of  ways,  so  that 
very  great  strength  is  obtained  without  a  very  great  increase 
of  bulk.  Such  methods  of  joining  timber  are  known  as 
scarfing  and  interlocking,  tonguing,  dovetailing,  mortis- 
ing, etc. 

FIG.  99. 


What  is  scarf- 
ing and  inter- 
locking? 


2G4.  Scarfing  and  interlocking  is  the  method 
of  insertion  in  which  the  ends  of  pieces  over- 
lay each  other j  and  are  indented  together,  so 
as  to  resist  the  longitudinal  strain  by  extension,  as  in  tie 
bearers  and  the  ends  of  hoops.  (See  Fig.  99.) 

265.  Tonguing  is  that  method  of  insertion  in  which  the 


118 


WELLS'S   NATURAL   PHILOSOPHY. 


What      is 
tonguing  ? 


What  is  dove- 
tailing ? 

which  the 


What  is  mor 
tising? 


FIG.  101. 


edges  of  boards  are  wholly,  or  partially  received 
by  channels  in  each  other. 

266.  Dovetailing  is   a  FIG.  100. 
method   of   insertion    in 

parts   are   connected   by  g 
wedge-shaped  indentations  which  per- 
mit them  to  be  separated  only  in  one 
direction.     (See  Fig.  100.) 

267.  Mortising  is  a  method  of  insertion  in 
which  the  projecting  extremity  of  one  timber  is 

received  into  a  perforation  in  another.  (See  Fig.  101.)  The 

opening  or  hole  cut  in 
one  piece  of  wood  to  re- 
ceive or  admit  the  pro- 
jecting extremity  of  an- 
other piece,  is  called  a 
mortise  ;  and  the  end  of 
the  timber  which  is  re- 
duced in  dimensions  so 
as  to  be  fitted  into  a  mor- 

tise, for  fastening  two  timbers  together,  is  called  a  tenon. 

268.  The  form  in  which  a  given  quantity  of 
matter  can  be  arranged  in  order  to  oppose  the 
greatest  resistance  to  a  bending  force,  is  that 
of  a  hollow  tube,  or  cylinder  ;  and  the  strength 
of  a  tube  is  always  greater  than  the  strength 

of  the  same  quantity  of  matter  made  into  a  solid  rod. 

The  most  beautiful  and  striking  illustrations  of  this  princi- 


In  what  form 
can  a  given 
quantity  of 
matter  be  ar- 
ranged to  op- 
pose the  great- 
est resistance  ? 


lustrations  of  pie  occur  in  nature.  The  bones  of  men  and  animals  are  hoi- 
this  principle  ?  loWj  and  neariy  cylindrical,  because  they  can  in  this  form, 
with  the  least  weight  of  material,  sustain  the  greatest  force.  The  stalks  of 
numerous  species  of  vegetables,  especially  the  grain-bearing  plants,  as  wheat, 
rice,  oats,  etc.,  which  are  required  to  bear  the  weight  of  the  ripened  ear  of 
grain,  or  seed,  are  hollow  tubes,  and  their  strength,  compared  with  their 
lightness,  is  most  remarkable.  In  this  form  they  not  only  sustain  the  crush- 
ing weight  of  the  ear  which  they  bear  at  the  summit,  but  also  the  force  of  the 
wind.  In  the  construction  of  columns  for  architectural  purposes,  especially 
those  made  of  metal,  this  principle  is  taken  advantage  of.* 
*  la  that  most  gigantic  -work  of  modern  engineering,  the  Britannia  Tubular  Bridge, 


MATERIALS   FOR   ARCHITECTURAL   PURPOSES.          119 

. 

269.  A  beam,  supported  at  its  two  ends,  when  bent  by  its 
bent5"  in  the  weight  in  the  middle,  has  its  liability  to  break  greatly  in- 
"li{ldlekJliabie  creased,  because  the  destroying  force  acts  with  the  advantage 
of  a  long  lever,  reaching  from  the  end  of  the  beam  to  the  cen- 
ter ;  and  the  resisting  force  or  strength  acts  only  with  the  force  of  a  short  lever 
from  the  side  to  the  center;  at  the  same  time,  a  little  only  of  the  beam  on  the 
under  side  is  allowed  to  resist  at  all. 

This  last  circumstance  is  so  remarkable,  that  the  scratch  of  a  pin  on  the 
under  side  of  a  beam,  resting  as  here  supposed,  will  sometimes  suffice  to  begin 
the  fracture. 

SECTION    II. 

APPLICATION  OP  MATERIALS  FOR  ARCHITECTURAL  OR  STRUCTURAL  PURPOSES. 

whatisArchi-        270.  Architecture,  in  its  general  sense,  is  the 
art  of  erecting  buildings.     In  modern  use,  the 
name  is  often  restricted  to  the  external  forms,  or  styles  of 
buildings. 

To  what  do  Tne  different  varieties  of  architecture  undoubtedly  owe  their 
the  different  origin  to  the  rude  structures  which  the  climate  or  materials  of 
I?cheitec8tarc°f  an^  countI7  obliged  its  early  inhabitants  to  adopt  for  tempo- 
probabiy  owe  rary  shelter.  These  structures,  with  all  their  prominent  fea- 
ongm*  tures,  have  been  afterward  kept  up  by  their  refined  and 
opulent  posterity.  Thus  the  Egyptian  style  of  architecture  had  its  origin  in 
the  cavern,  or  mound.  The  Chinese  architecture  is  modeled  from  a  tent ;  the 
Grecian  is  modeled  from  the  wooden  cabin ;  and  the  Gothic,  it  has  been  sug- 
gested, from  the  bower  of  trees. 

on  what  does  271.  The  strength  of  a  building  will  princi- 
abuudinfphri°nf-  Pa%  depend  on  the  walls  being  laid  on  a  good 
dpaiiy depend?  and  £rm  foundation,  of  sufficient  thickness  at 
the  bottom,  and  standing  perfectly  perpendicular.  Its 
usefulness  will  depend  upon  a  proper  arrangement  of  its 
parts. 

crossing  the  Menai  Straits,  which  separate  the  island  of  Anglesea  from  the  mainland  of 
Great  Britain,  advantage  has  been  taken  of  the  strength  of  matter  arranged  in  the  form 
of  a  tube  or  hollow  cylinder.  The  entire  bridge  is  formed  of  immense  rectangular  tubes 
of  iron,  26  feet  high  in  the  canter,  14  feet  wide,  and  having  an  entire  length  of  1513  feet, 
with  an  elevation  above  the  water  of  more  than  100  feet.  The  sides  of  the  tubes  are  also 
composed  of  smaller  tubes,  united  together  in  a  peculiar  manner,  so  as  to  obtain  the 
maximum  of  strength  from  the  form  of  structure ;  and  so  great  is  this  strength,  that  a 
train  of  loaded  cars,  weighing  280  tons,  and  impelled  with  great  velocity,  deflects  the 
tubes  in  their  centers  less  than  three  fourths  of  an  inch.  The  entire  weight  of  the  tubes 
composing  this  bridge  is  upward  of  10,500  tons,  the  length  of  two  of  the  spans,  or  distances 
between  the  points  of  support,  being  460  feet  each.  The  same  amount  of  iron  in  the  form 
of  a  solid  rod  or  beam,  would  not  probably  have  sustained  its  own  weight. 


120  WELLS'S   NATURAL   PHILOSOPHY. 

ne?  ^^*  ^  PILE>  m  architecture  and  engineer- 
ing, is  a  cylinder  of  wood  or  metal  pointed  at 
one  extremity,  and  driven  forcibly  into  the  earth,  to  serve 
as  a  support  or  foundation  of  some  structure.  It  is  gen- 
erally used  in  marshy  or  wet  places,  where  a  stable  found- 
ation could  not  otherwise  be  obtained. 

In  constructing  columns  for  the  support  of  the  various  parts 
umns  support-      of  a  building,  or  of  great  weights,  they  are  made  smaller  at 

lafUr  *ati8the  the  toP  than  at  the  bottom>  because  the  lower  part  of  the 
bottom  than  column  must  sustain  not  only  the  weight  of  the  superior  part, 
the  top?  kut  ^0  the  weight  which  presses  equallj'-  on  the  whole 

column.  Therefore  the  thickness  of  the  column  should  gradually  decrease 
from  bottom  to  top. 

what  is  an        273.  An  ARCH  is  a  concave  or  hollow  struct- 
arch?        ure^  generally  of  stone  or  brick,  supported  by 
its  own  curve. 

The  base  of  an  arch  is  supported  by  the  support  upon  which  it  rests,  while 
all  the  other  parts  constituting  the  curve  are  sustained  in  their  positions  by 
their  mutual  pressure,  and  by  the  adhesion  of  the  cement  interposed  between 
their  surfaces. 

A  continued  arch  is  termed  a  vault. 

An  arch  is  capable  of  resisting  a  much  greater  amount  of 
Why  is  an  arch  A    , 

stronger    than     pressure  than  a  horizontal  or  rectangular  structure  constructed 

a  horizontal  of  tiie  same  materials,  because  the  arrangement  of  the  mate- 
rials composing  the  arch  is  such,  that  the  force  which  would 
break  a  horizontal  beam  or  structure  is  made  to  compress  all  the  particles  of 
the  arch  alike,  and  they  are  therefore  in  no  danger  of  being  torn  or  overcome 
separately. 

274.  The  vertical  wall  which  sustains  the  base  of  an  arch 
'i»  termed  an   abutment:    when   there  are  two  contiguous 
arches,  the  intermediate  supporting  wall  is  called  a  pier. 
What  ar    H  "^  beautiful  application  of  the  principles  of  the  arch  exists 

lustrations  of  in  the  human  skull,  protecting  the  brain.  The  materials  are 
oftfie  arch?6*  lier6  arranoed  m  such  a  way  as  to  afford  the  greatest  strength 
with  the  least  weight.  The  shell  of  an  egg  is  constructed 
upon  the  principle  of  the  arch ;  and  it  is  almost  impossible  to  break  an  egg 
with  the  hands,  by  pressing  directly  upon  its  ends.  A  thin  watch-glass,  for 
the  same  reason,  sustains  great  pressure.  A  dished  or  arched  wheel  of  a 
carriage  is  many  times  stronger  to  resist  all  kinds  of  shocks  than  a  perfectly 
flat  wheel.  A  full  cask  may  fall  without  damage,  when  a  strong  square  box 
would  be  dashed  to  pieces. 

what  is  an  275.  By  an  order  in  architecture  we  under- 
tecturc'?a"  stand  a  certain  mode  of  arranging  and  decor- 


MATERIALS   FOR   ARCHITECTURAL   PURPOSES.         121 

ating  a  column,  and  the  adjacent  parts  of  the  structure 
which  it  supports  or  adorns. 

HOW  many  or-  276.  Five  orders  are  recognized  in  architec- 
ture ""are  ^ure — ^e  Doric,  Ionic,  and  Corinthian,  de-' 
there?  rived  from  the  Greeks  ;  to  these  the  Romans 

added  two  others,  known  as  the  Tuscan  and  Composite, 
what  is  a  Pi-        277.  A  Pilaster  is  a  square  column  gener- 
ally set  within  a  wall,  and  not  standing  alone, 
what  is  a  For-        278.  A  Portico  is  a  continued  range  of  col- 
umns, covered  at  the  top  to  shelter  from  the 
weather. 

What  are  Bai-  279.  Balusters  are  small  columns,  or  pillars 
usters?  of  W00(j?  stone,  etc.,  used  in  terraces  or  tops 
of  buildings  for  ornament ;  also  to  support  a  railing.  When 
continued  for  some  distance,  they  form  a  balustrade. 
into  what  two  280.  An  order,  in  architecture,  consists  of 
oSb?nbarchT-  two  principal  members — the  column  •  and  the 
tecture.diyided?. entablature—  each  of  which  is  divided  into 
three  principal  parts. 

what  is  the  281.  The  Entablature  is  the  horizontal  con- 
Entabiature?  tinuous  portion  which  rests  upon  a  row  of 
columns. 

Into  how  many         It  is  divided  into  the  architrave,  which  is  the  lower  part  of 

parts  is  the  En-     flie  Entablature:  the  frieze,  which  is  the  middle  part;  and 
tablature      di- 
vided-?         .      .the  cornice,  which  is  the  upper,  or  projecting  part 

282.  The  column  is  divided  into  the  base. 

In  to  how  many  4 

parts   ia  the    the  shaft,  and  the  capital. 

column  divided  ? 

The  base  is  the  lower  part,  distinct  from  the  shaft ;  the 

shaft  is  the  middle,  or  longest  part  of  the  column ;  the  capital  is  the  upper,  or 
ornamental  part  resting  on  the  shaft. 

,  The  height  of  a  column  is  always  measured  in  diameters  of  the  column 
itself,  taken  at  the  base  of  the  shaft.  Thus  we  say  the  height  of  the  Doric, 
column  is  six  times  its  diameter,  and  the  height  of  the  Corinthian,  ten  diam-- 
eters.  Fig.  102  represents  the  various  parts  of  an  order  of  architecture. 

what  is  the          283.  The  Fagade  of  a  building  is  its  whole 

Facade    of    a       -fVrmf 
Building? 

Architecture  ought  to  be  considered  as  a  useful,  and  not  as 
a  fine  art.  It  is  degrading  the.  fine  arts  to  make  them  entirely  subservient  to 
utility.  It  is  out  of  taste  to  make  &  statue  of  Apollo  hold  a  candle,  or  a  fine 

6 


122 


WELLS'S   NATURAL  PHILOSOPHY. 


painting  stand  as  a  fire-board.  Our  houses  are  for  use,  and  architecture  is, 
therefore,  one  of  the  useful  arts.  In  building,  we  should  plan  the  inside  first, 
and  then  the  outside  to  cover  it.  It  is  in  bad  taste  to  construct  a  dwelling- 
house  in  the  form  of  a  Grecian  temple,  because  a  Grecian  temple  was  intended 
for  external  worship,  not  for  a  habitation,  or  a  place  of  meeting.* 


FIG.  102. 


Entablature 


Column. 


Stylobate,    or   Pe-  ^ 
dental... 


j :,  j. 


Cornice. 

Frieze. 

Architrave. 

Capital 


Shaft. 


Base. 

Cornice. 


Die. 

Plinth. 


How  may  an 
estimate  of  the 
durability  of 
•tone  for  archi- 
tectural pur- 
poses be  made  ? 


284.  In  selecting  a  stone  for  architectural  purposes,  we  may 
be  able  to  form  an  opinion  respecting  its  durability  and  per- 
manence. By  visiting  the  locality  from  whence  it  was  ob- 
tained, we  may  judge  from  the  surfaces  which  have  been  long 
exposed  to  the  weather  if  the  rock  is  liable  to  yield  to  atmos- 
pheric influences,  and  the  conditions  under  which  it  does  so.  For  example, 
if  the  rock  be  a  granite,  and  it  be  very  uneven  and  rough,  it  may  be  inferred 
that  it  ia  not  very  durable;  that  the  feldspar,  which  forms  one  of  its  compo- 

*  Prof.  Henry. 


HYDROSTATICS.  123 

nent  parts,  is  more  readily  decomposed  by  the  action  of  moisture  and  frost 
than  the  quartz,  which  is  another  ingredient ;  and  therefore  it  is  very  unsuit- 
able for  building  purposes.  Moreover,  if  it  possess  an  iron-brown  or  rusty 
appearance,  it  may  be  set  down  as  highly  perishable,  owing  to  the  attraction 
which  this  iron  has  for  oxygen,  causing  the  rock  to  increase  in  bulk,  and  so 
disintegrate. 

Sandstones,  termed  freestones,  are  ill  adapted  for  the  external  portions  of 
exposed  buildings,  because  they  readily  absorb  moisture;  and  in  countries 
where  frosts  occur,  the  freezing  of  the  water  on  the  wet  surface  continually 
peels  off  the  external  portions,  and  thus,  in  time,  all  ornamental  work  upon 
,the  stone  will  be  defaced  or  destroyed. 


CHAPTER    VIII 

HYDROSTATICS. 

what  is  the        285.  HYDROSTATICS  is  that  depal 

science  of  Hy-  m  *• 

drostatics?  Physical  Science  which  treats  of  the  weight, 
pressure,  and  equilibrium  of  water,*  and  other  liquids  at 
rest. 

*  Water  is  a  fluid  composed  of  oxygen  and  hydrogen,  in  the  proportion  of  8  parts  of 
oxygen  to  1  of  hydrogen.  It  is  one  of  the  most  abundant  of  all  substances,  constituting 
three  fourths  of  the  weight  of  living  animals  and  plants,  and  covering  about  three-fifths 
of  the  earth's  surface,  in  the  form  of  oceans,  seas,  lakes,  and  rivers. 

In  the  northern  hemisphere  the  proportion  of  land  to  water  is  as  419  to  1000;  while  in 
the  southern  hemisphere  it  is  as  129  to  1000.  The  maximum  depth  of  the  ocean  has  never 
been  ascertained.  Soundings  were  obtained  in  the  South  Atlantic  in  1853,  between  Rio 
Janeiro  and  the  Cape  of  Good  Hope,  to  the  depth  of  48,000  feet,  or  about  9  miles.  Other 
soundings,  made  during  the  recent  U.  S.  survey  of  the  Gulf  Stream,  extended  to  the 
depth  of  34,200  feet  without  finding  bottom.  The  average  depth  of  the  ocean  has  been 
estimated  at  about  2000  fathoms. 

Notwithstanding  this  apparent  immensity  of  the  ocean,  yet,  compared  with  the  whole 
bulk  of  the  earth,  it  is  a  mere  film  upon  its  surface  ;  and  if  its  depth  were  represented  on 
an  ordinary  globe,  it  would  hardly  exceed  the  coating  of  varnish  placed  there  by  the 
manufacturer. 

The  source  of  all  our  terrestrial  waters  is  the  ocean.  By  the  action  of  evaporation  upon 
its  surface,  a  portion  of  its  water  is  constantly  rising  into  the  atmosphere  in  the  form  of 
vapor,  which  again  descends  in  the  form  of  rain,  dew,  fog,  etc.  These  waters  combine  to 
form  springs  and  rivers,  which  all  at  last  discharge  into  the  ocean,  the  point  from  which 
they  originally  came,  thus  forming  a  constant  round  and  circulation.  "  All  the  rivers 
run  into  the  sea,  yet  the  sea  is  not  full,"  because  the  quantity  of  water  evaporated  from 
the  sea  exactly  equals  the  quantity  poured  into  it  by  the  rivers.  In  nature,  water  is 
never  found  perfectly  pure ;  that  which  descends  as  rain  is  contaminated  by  the  impuri- 
ties it  washes  out  of  the  air ;  that  which  rises  in  springs  by  the  substances  it  meets  with 
in  the  earth.  Any  water  which  contains  less  than  fifteen  grains  of  solid  mineral  matter  in 
a  gallon,  is  considered  as  comparatively  pure.  Some  natural  waters  are  known  so  pure 
that  they  contain  only  l-20th  of  a  grain  of  mineral  matter  to  the  gallon,  but  such  instances 
are  very  rare.  Water  obtained  from  different  sources  may  be  classed,  as  regards  com- 


124  WELLS'S   NATURAL   PHILOSOPHY. 


286'  Liquids  Lave  but  a  slight  degree  of 
elastic?  compressibility   and    elasticity,    as   compared 

with  other  bodies. 

What  r  iiius  ^^'  The  elasticity  of  water  may  be  shown  in  various  way  s. 
trations  of  the  When  a  flat  stone  is  thrown  so  as  to  strike  the  surface  of 
elasticity  of  wa-  water  neariy  horizontally,  or  at  a  slight  angle,  it  rebounds 
with  considerable  force  and  frequency.  Water  also  dashed 
against  a  hard  surface  shows  its  elasticity  by  flying  off  in  drops  in  angular 
directions.  Another  familiar  example  of  the  elasticity  of  water  is  observed, 
when  we  attempt  to  separate  a  drop  of  water  attached  to  some  surface  for 
which  it  has  a  strong  attraction.  The  drop  will  elongate,  or  allow  itself  to  be 
drawn  out  to  a.  considerable  degree,  before  the  cohesion  of  its  constituent  par- 
ticles is  wholly  overcome  ;  and  if  the  separating  force  is  at  any  time  relaxed, 
or  discontinued,  the  elasticity  of  the  water  will  restore  the  drop  to  very  nearly 
its  original  form  and  position.  Mercury  is  much  more  elastic  than  water,  and 
rebounds  from  a  reflecting  surface  with  considerable  velocity  and  violence. 
The  exercise  of  both  the  elastic  and  compressive  principle  is,  however,  so  ex- 
tremely limited  in  liquids,  that  for  all  practical  purposes  this  form  of  matter  is. 
regarded  as  inelastic  and  uncompressible  ;  or,  in  other  words,  the  elasticity 
and  compressibility  of  water  produce  no  appreciable  effects. 
To  what  ex  ^e  compressibility  of  water  is  not  so  easily  demonstrated 

tent  has  water  as  is  its  elasticity,  although  the  elasticity  is  a  direct  conse- 
pre6fled°?m~  quent  of  the  compressibility.  An  experiment  of  Mr.  Perking 

showed  that  water,  under  a  pressure  of  15,000  pounds  to  the 
square  inch,  was  reduced  in  bulk  1  part  in  24. 

In  what  man  ^^'  ~^n  ^u^  bodies,  as  has  been  already  shown  (§§  34, 

ner  do  the  par-  36),  the  attractive  and  repulsive  forces  existing  between  the 
hdes  of  liquids  particles  are  so  nearly  balanced,  that  the  particles  move  upon 
each  other  ?  each  other  with  the  greatest  facility.  The  particles  which 
make  up  a  collection  of  fine  sand,  or  dust,  also  move  upon 
each  other  with  great  facility  :  but  the  particles  of  a  liquid  possess  this  addi- 
tional quality,  viz.,  that  of  moving  upon  themselves  without  friction.  The 
particles  of  no  solid  substance,  however  fine  they  may  be  rendered,  possess 
this  property. 

289.  From  this  is  derived  a  great  fundamental  principle  lying  at  the  basis 
of  all  the  mechanical  phenomena  connected  with  liquid  bodies,  viz.  :  — 

parative  purity,  as  follows  ;  Rain  water  must  be  considered  as  the  purest  natural  watc", 
especially  that  which  falls  in  districts  remote  from  towns  or  habitations  ;  then  comes 
river  water  ;  next,  the  water  of  lakes  and  ponds  ;  next,  spring  waters  ;  and  then  the 
waters  of  mineral  springs.  Succeeding  these,  are  the  waters  of  great  arms  of  the  ocean, 
into  which  immense  rivers  discharge  their  volumes,  as  the  water  of  the  Black  Sea,  which 
is  only  brackish  ;  then  the  waters  of  the  ocean  itself:  then  those  of  the  Mediterranean 
and  other  inland  seas  ;  and  last  of  all,  the  waters  of  those  lakes  which  have  no  outlet,  as 
the  Dead  Sea,  Caspian,  Great  Salt  Lake  of  Utah,  etc.  etc. 

All  natural  waters  contain  air,  and  sometimes  other  gaseous  substances.  Fishes  and 
other  marine  animals  are  dependent  upon  the  air  which  water  contains  for  their  respira- 
tion and  existence.  It  is  owing  to  the  presence  of  aix  in  water  that  it  sparkles  and 
bubbles. 


HYDROSTATICS. 


125 


What  great  law- 
constitutes  the 
basis  of  all  the 
mechanical 
phenomena  of 
liquids  ? 


FIG.  103. 


290.  Liquids   transmit  pressure  equally  in 
all  directions. 

This  remarkable  property  constitutes  a  very  characteristic 
distinction  between  solids  and  liquids ;  since  solids  transmit 
pressure  only  in  one  direction,  viz.,  in  the  line  of  the  direction 

of  the  force  acting  upon  them,  while  liquids  press  equally  in  all  directions, 

upward,  downward,  and  sideways. 

In  order  to  obtain  a  clear 
Illustrate    the  ,.  _    ,,  .     . 

equality     of        understanding    of   the    princi- 

Sids?Ure  iU  Uq~     ple  of  the  e(luality  of  pressure 
in  liquids,    let   us  suppose  a 

vessel,  Fig.  103,  of  any  form,  in  the  sides  of 

which  are  several  tubular  openings,  ABC 

D  E,   each  closed  by  a  movable  piston.      If 

now  we  exert  upon  the  top  of  the  piston  at 

A,  a  downward  pressure  of  20  pounds,  this 

pressure  will  be  communicated  to  the  water, 

which  will  transmit  it  equally  to  the  internal 

face  of  all  the  other  pistons,  each  of  which 

will  be  forced  outward  with  a  pressure  equal  to  20  pounds,  provided  their 

surfaces  in  contact  with  the  water  are  each  equal  to  that  of  the  first  piston. 

.But  the  same  pressure  exerted  on  the  pistons  is  equally  exerted  upon  all  parts 

of  the  sides  of  the  vessel,  and  therefore  a  pressure  of  20  pounds  upon  a  square 

inch  of  the  surface  of  the  piston  A,  will  produce  a  pressure  of  20  pounds  upon 

every  square  inch  of  the  interior  of  the  surface  of  the  vessel  containing  the 

liquid. 

FiG.  104.  The  same  principle  may 

also  be  shown  by  another 
experiment.  Suppose  a 
cylinder,  Fig.  104,  in  which 
a  piston  is  fitted,  to  termi- 
nate in  a  globe,  upon  the 
sides  of  which  are  little 
tubular  openings.  If  the 
globe  and  the  cylinder  are 
filled  with  water,  and  the 
piston  pressed  down,  the 

liquid  will  jet  out  equally  from  all  the  orifices,  and  not  solely  from  the  one 

which  is  in  a  direct  line  with,  and  opposite  to  the  piston. 

291.  This  property  of  transmitting  pressure  equally  and 
ner  may  a  Hq-     freely  in  every  direction,  is  one  in  virtue  of  which  a  liquid 
machine  f*8  &     becomes  a  machine,  and  can  be  made  to  receive,  distribute, 

and  apply  power.  Thus,  if  water  be  confined  in  a  vessel, 
and  a  mechanicti  force  exerted  on  any  portion  of  it,  this  force  will  be  at  once 
transmitted  throughout  the  entire  mass  of  liquid. 


126 


WELLS'S  NATURAL  PHILOSOPHY. 


What  is  the  Hy- 
drostatic  Para- 


FIG  105 


How   do 
hTthe 


the 


^e  en"ects  of  the  practical  application  of  this  principle  are 
So  remarkable  that  it  has  been  called  the  Hydrostatic  Para- 
dox, since  the  weight,  or  force,  of  one  pound,  applied  through 
the  medium  of  an  extended  surface  of  some  liquid,  may  be  made  to  produce 
a  pressure  of  hundreds,  or  even  thousands  of  pounds.  Thus,  in  Fig.  105,  A 
an<^  a  are  ^wo  cy^n(^ers  containing  water  connected 
by  a  pipe,  each  fitted  with  a  piston  in  such  a  way  as 
f  rm  to  render  the  whole  a  close  vessel.  Suppose  the 

[fl  3?  ra  PI  II  i  area  of  the  base  of  the  piston,  p,  to  be  one  square 
j  J  _  1  i^B$  ^  inch,  and  the  area  of  the  base  of  the  piston,  P,  to  be 
1,000  square  inches.  Now  any  pressure  applied  to 
the  small  piston  will  be  transmitted  by  the  water  to 
the  large  piston  ;  so  that  every  portion  of  surface  in 
the  large  piston  will  be  pressed  upward  with  the 
same  force  that  an  equal  portion  of  the  surface  in  the  small  piston  is  pressed 
downward.  A  pressure,  therefore,  of  1  pound  acting  on  the  base  of  the  pis- 
ton jj,  will  exert  an  outward  pressure  of  1,000  pounds  acting  on  the  base  of 
the  piston  P  ;  so  that  a  weight  of  1  pound  resting  upon  the  piston  p,  would 
support  a  weight  of  1,000  pounds  resting  upon  the  piston  P. 

The  action  of  the  forces  here  supposed  differs  in  nothing 
from  that  of  like  forces  acting  on  a  lever  having  unequal 
anna  in  the  proportion  of  1  to  1,000.  A  weight  of  1  pound 
acting  on  the  longer  arm  of  such  a  lever,  would  support,  or 
raise  a  weight  of  1,000  pounds  acting  on  the  shorter  arm. 
rp^  liquid  contained  in  the  vessel,  in  the  present  case,  acts 
as  the  lever,  and  the  inner  surface  of  the  vessel  containing 
it  acts  as  the  fulcrum.  If  the  piston  p  descends  one  inch,  a  quantity  -of 
water  which  occupies  one  inch  of  the  cylinder  a  will  be  expelled  from  it,  and 
as  the  vessel  A  a  is  filled  in  every  part,  the  piston  P  must  be  forced  upward 
until  space  is  obtained  for  the  water  which  has  been  expelled  from  the  cylin- 
der a.  But  as  the  sectional  area  of  A  is  1,000  times  greater  than  that  of  a, 
the  height  through  which  the  piston  P  must  be  raised  to  give  this  space,  will 
be  1,000  times  less  than  that  through  which  the  piston  p  has  descended. 
Therefore,  while  the  weight  of  1  pound  on  p  has  moved  through  1  inch,  the 
weight  of  1,000  pounds  on  P  will  be  raised  through  only  l-l,000th  part  of  an 
inch.  If  this  process  were  repeated  a  thousand  times  the  weight  of  1,000 
pounds  on  P  would  be  raisod  through  1  inch  ;  but  in  accomplishing  this,  the 
weight  of  1  pound  acting  on  P  would  be  moved  successively  through  1,000 
inches.  The  mechanical  action,  therefore,  of  the  power  in  this  case,  is  ex- 
pressed by  the  force  of  1  pound  acting  successively  through  1,000  inches, 
while  the  mechanical  effect  produced  upon  the  resistance  is  expressed  by  1,000 
pounds  raised  through  1  inch. 
What  is  a  Hy-  292'  Tn6  HYDRAULIC,  OF  HYDROSTATIC 

drauiic  Press?    PRESS,  is  a  machine  arranged  in^uch  a  man- 
ner, that  the  advantages  derived  from  the  principle  that 


the  forces  act- 
ing  on  the  arms 


HYDROSTATICS. 


127 


liquids  transmit  pressure  equally  in  all  directions,  may  be 
practically  applied. 

The  principle  of  the  construction  and  action  of  the  hydraulic  press  is  ex- 
plained in  the  preceding  paragraph  (§  291),  and  Fig.  105,  represents  a  section 
of  its  several  parts.  FlG>  106> 


Fig.  106  represents  the  hydraulic  press  as  constructed  for  practical  purposes. 
In  a  small  cylinder,  A,  the  piston  of  a  forcing-pump,  P,  works  by  means  of 
the  handle  M.  The  cylinder  of  the  forcing-pump,  A,  connects,  by  means  of  a 
tube,  K,  leading  from  its  base,  with  a  large  cylinder,  B.  In  this  moves  also 
a  piston,  P,  having  its  upper  extremity  attached  to  a  movable  iron  plate, 
which  works  freely  up  and  down  in  a  strong  upright  frame-work,  Q.  Be- 
tween this  plate  and  the  top  of  the  frame-work  the  substance  to  be  pressed  is 
placed.  To  operate  the  press,  water  is  raised  in  the  forcing-pump,  A,  by 
raisins:  the  handle  M,  from  a  small  reservoir  beneath  it,  a;  by  depressing  the 
handle,  the  water  filling  the  small  cylinder  A  is  forced  through  a  valve.  H, 
and  the  pipe  K,  into  the  larger  cylinder  B,  where  it  acts  to  raise  the  larger 
piston,  and  causes  it  to  exert  its  whole  force  upon  the  object  confined  be- 
tween the  iron  plate  and  the  top  of  the  frame- work.  If  the  area  of  the  baso 
of  the  piston  p  is  a  square  inch  in  diameter,  and  the  area  of  the  base  of.  the 
piston  P  1,000  square  inches,  then  a  downward  pressure  of  one  pound  on  p 
will  exert  an  upward  pressure  of  1,000  pounds  on  P. 


128 


WELLS'S   NATURAL   PHILOSOPHY. 


as      well      as 
downward  ? 


PIG.  107. 


.As  thus  constructed,  the  hydraulic  press  constitutes  the  most  powerful 
mechanical  engine  with  which  we  are  acquainted,  the  limits  to  its  power 
being  bounded  only  by  the  strength  of  the  machinery  and  material.  By 
means  of  this  press,  cotton  is  pressed  into  bales,  ships  are  raised  from  the 
water  for  repair,  chain-cables  are  tested,  etc.  etc. 

will  liquids  293.  As  liquids  transmit  pressure  equally  in 
press  upward  ajj  Directions,  it  follows  that  any  given  portion 
of  a  liquid  contained  in  a  vessel  will  press  up- 
ward upon  the  particles  above  it,  as  powerfully  as  it 
presses  downward  upon  the  particles  below  it. 

How  is  the  u  -  Tllis  fact  may  be  ilmstrated  D7  means  of 
ward  pressure  the  apparatus  represented  in  Fig.  107.  If 

by  eTpe^meTt?     a  Plate  °f  meta1'  B»  be  held  against  the  bot' 
tom  of  a  glass  tube,  g,  by  means  of  a  string, 

v,  and  immersed  in  a  vessel  of  water,  the  water  being  up  to 
the  level  n  n.  the  plate  B  will  be  sustained  in  its  place  by  the 
upward  pressure  of  the  water;  to  show  that  this  is  the  case, 
it  is  only  necessary  to  pour  water  into  the  tube  <?,  until  it 
rises  to  the  level  n  n,  when  the  plate  will  immediately  fall, 
the  upward  pressure  below  the  plate  B  being  neutralized 
by  the  downward  pressure  of  the  water  in  the  tube  g. 

"  Some  persons  find  it  difficult  to  understand  why  there 
should  be  an  upward  pressure  in  a  mass  of  liquid,  as  well 

as  a  downward  and  lateral  pressure.     But  if  in  a  mass  of  

liquid  the  particles  below  had  not  a  tendency  upward  equal 
to  the  weight,  or  downward  pressure  of  the  particles  of  liquid  above  them, 
they  could  not  support  that  part  of  the  liquid  which  rests  upon  them.     Their 
tendency  upward  is  owing  to  the  pressure  around  them  from  which  they  are 
trying  to  escape."* 

294-  The   pressure  exerted  by  a 
column  of  liquid  is  proportioned  to, 
or  measured  by   the  height  of  the 
column,  and   not   by  its  bulk,   or 
quantity. 

If  we  take  a  tube  in  the  form  of  the  letter  TJ,  with  one  of  its 
branches  much  smaller  than  the  other,  as  in  Fig.  108,  and  pour 
water  into  one  of  the  branches,  we  shall  find  that  the  liquid 
will  stand  at  the  same  height  in  both  tubes.  The  great  mass 
of  liquid  contained  in  the  large  tube,  A,  exerts  no  more  press- 
ure on  the  liquid  contained  in  the  small  tube,  D,  than  would  a 
smaller  mass  contained  in  a  tube  of  the  same  dimensions  as  D. 
And  if  A  contained  10,000  times  the  quantity  of  water  that  D 
contained,  the  water  would  rise  to  no  greater  elevation  in  D 

than  in  A. 

•  Arnott. 


To  what  is  the 
pressure  of  a 
column  of  liq- 
uid propor- 
tional ? 


FIG.  108. 


HYDROSTATICS. 


129; 


What  is  the 
principle    and 
action    of    the 
Hydrostatic 
Bellows  ? 


FlQ.  109. 


The  principle  that  the  pressure  exerted  by  a  column  of 
water  is  as  its  height,  and  not  as  its  quantity,  may  be  also 
illustrated  by  the  Hydrostatic  Bellows,  Fig.  109.  This  con- 
sists of  two  boards,  B  C  and  D  E,  united  together  by  means 
of  cloth,  or  leather,  A,  as  in  a  common  bellows.  A  small  ver- 
tical pipe,  T,  attached  to  the  side  communicates 
with  the  interior  of  the  bellows.  Heavyweights, 
W  W,  are  placed  upon  the  top  of  the  bellows 
when  empty.  If  water  be  poured  into  the  verti- 
cal pipe,  the  top  of  the  bellows,  with  the  weights 
upon  it,  will  be  lifted  up  by  the  pressure  of  the 
water  beneath ;  and  as  the  height  of  the  column 
of  water  increases,  so  in  like  proportion  may  the 
weights  upon  the  top  of  the  bellows  be  increased. 
It  is  a  matter  of  no  consequence  what  may  be  the 
diameter  of  the  vertical  tube,  since  the  power  of 
the  apparatus  depends  upon  the  height  of  the  col- 
umn of  water  in  the  small  tube,  and  the  area  of 
the  board,  B  C  ;  that  is,  the  weight  of  a  small  col- 
umn  of  water  in  the  vertical  pipe,  T,  will  be  capable 
of  supporting  a  weight  upon  the  board,  B  C,  greater 
than  the  weight  of  the  water  in  the  pipe,  in  the  same 
D  proportion  as  the  area  of  board  B  C  is  greater  than 
the  sectional  area  of  the  bore  of  the  pipe.  Thus,  if 
the  area  of  the  bore  of  the  pipe  be  a  quarter  of  an  inch,  and  the  area  of  the 
board  forming  the  top  of  the  bellows  a  square  foot,  then  the  proportion  of  the 
pipe  to  the  board  will  be  that  of  576  to  1 ;  and,  consequently,  the  weignt 
capable  of  being  supported  by  the  board  will  be  576  times  pIG>  no. 
the  weight  of  the  water  contained  in  the  pipe. 

In  this  manner  a  strong  cask,  a,  Fig.  110, 
filled  with  liquid,  may  be  burst  by  a  few 
ounces  of  water  poured  into  a  long  tube,  b  c, 
communicating  with  the  interior  of  the  cask. 
This  law  of  pressure  is  sometimes  exhibited 
on  a  great  scale  in  nature,  in  the  bursting  of  rocks,  or  mount- 
ains. Suppose  a  long  vertical  fissure,  as  in  Fig.  Ill,  to  com- 
municate with  an  internal  cavity  formed  in  a  mountain,  with- 
out any  outlet.  Now,  when  the  fissure  and  cavity  become 
filled,  an  enormous  pressure  is  exerted,  sufficient,  it  may  be, 
to  crack,  or  disrupture,  the  whole  mass  of  the  mountain. 

The  most  striking  effects  of  the  pressure  of  the  water  at 
great  depths  are  exhibited  in  the  ocean.  If  a  strong,  square 
glass  bottle,  empty  and  firmly  corked,  be  sunk  in  water,  its 
sides  are  generally  crushed  in  by  the  pressure,  before  it  has 
reached  a  depth  of  60  feet.  Divers  plunge  with  impunity  to 
certain  depths,  but  there  is  a  limit  beyoad  which  they  can  not  sustain  thu 

6* 


What  are  il- 
lustrative ex- 
amples of  the 
pressure  of 
liquids? 


130 


WELLS'S  NATURAL  PHILOSOPHY. 


FiG.  111. 


immense  pressure  on  the  body 
exerted  by  the  water.  It  is  prob- 
able, also,  that  there  is  a  limit  of 
depth  beyond  which  each  spe- 
cies of  fish  can  not  live.  The 
principle  of  the  equal  transmis- 
sion of  pressure  by  liquids,  how- 
ever, enables  fishes  to  sustain  a 
very  great  pressure  of  water 
without  being  crushed  by  it; 
the  fluids  contained  within  them 
pressing  outward  with  as  great  a 
force  as  the  liquid  which  sur- 
rounds them  presses  inwards. 
When  a  ship  founders  at  sea,  the  great  pressure  at  the  bottom  forces  the 

water  into  the  pores  of  the  wood,  and  increases  its  weight  to  such  an  extent 

that  no  part  can  ever  rise  again. 

upon  what  does  295.  The  pressure  upon  the  bottom  of  a  vessel 

Son  tpheesbuorte-  containing  a  liquid,  is  not  effected  by  the  shape 

contataing6^!  °f  *^e  vessel,  but  depends  solely  upon  the  area 

md  depend?  o£  fae  \)a&Q}  and  its  depth  below  the  surface. 

This  arises  from  the  law  of  equal  distribution  of  pressure  in  liquids.     Fig. 

112  represents  two  different  vessels 
having  equal  bases,  and  the  same  per- 
.P....I      L..CJ  pendicular  depth  of  water  in  thenv 
i  Although  the  quantity  of  water  con- 
:  tained  in  one  is  much  greater  than  in 
the  other,  the  pressure  sustained  by 
these  bases  will  be  thesame. 

In  a  conical  vessel,  Fig.  113,  the 

base,  C  D,  sustains  a  pressure  measured  by  the  height  of  the  column,  ABC 
D  ;  for  all  the  rest  of  the  liquid  only  presses  on  A  B  C  D  laterally,  and  resting 

FiG.  113.  FiG.  114. 


FIG.  112. 


on  the  sides,  E  C  and  F  D,  can  not  contribute  any  thing  to  the  pressure  on 
the  base,  0  D.    But  in  a  conical  vessel,  of  the  shape  represented  in  Fig.  114, 


HYDROSTATICS. 


131 


How  can  we 
calculate  the 
pressure  upon 
the  bottom  of 
a  vessel  con- 
taining water  ? 


the  pressure  on  A  B  a  portion  of  the  base,  E  F,  is  measured  by  the  column 
A  B  C  D  as  before ;  but  the  other  portions  of  the  liquid  not  resting  on  the 
gides  also  press  upon  the  bottom,  E  F ;  and  as  thq  pressure  of  the  column  A 
B  C  D  is  transmitted  equally,  every  portion  of  the  base,  E  F,  sustains  an 
equal  pressure  as  that  portion  of  the  base,  A  B,  which  is  directly  beneath  the 
column,  A  B  C  D ;  therefore  the  whole  pressure  on  the  base,  E  F,  is  the 
same  as  if  the  vessel  had  been  cylindrical,  and  filled  throughout  to  the  height 
indicated  by  the  dotted  lines,  G  H. 

296.  Hence,  to  find  the  pressure  of  water  upon  the  bottom  of  any  vessel, 
we  have  the  following  rule  : 

297.  Multiply  the  area  of  the  base  by  the 
perpendicular  depth  of  the  water,  and  this 
product  by  the  weight  of  a  cubic  foot  of 
water.  * 

Thus,  suppose  the  area  of  the  base  of  a  vessel  to  be  2  square  feet,  and  the 
perpendicular  depth  of  the  water  to  be  3  feet;  required  the  pressure  on 
the  bottom  of  the  vessel,  the  weight  of  a  cubic  foot  of  water  being  assumed 
to  be  1,000  ounces  (see  §  82). 

2X3==G  cubic  feet 

0X1,000=6,000  oz.=pressure  on  the  base  of  the  vessel 

*  "  The  actual  pressure  of  water  may  also  be  calculated  from  the  following  data.  It  is 
ascertained  that  the  weight  of  a  cubic  inch  of  water  of  the  common  temperature  of  62* 
Fahrenheit,  is  a  portion  of  a  pound  expressed  by  the  decimal  0-036065.  The  pressure, 
therefore,  of  a  column  of  water  one  foot  high,  having  a  square  inch  for  its  base,  will  be 
found  by  multiplying  this  by  12,  and  consequently  will  be  0-4328  Ib. 

"The  pressure  produced  upon  a  square  foot  by  a  column  one  foot  high,  will  be  found 
by  multiplying  this  last  number  by  144,  the  number  of  square  inches  forming  a  square 
fx>t ;  it  will  therefore  be  02-3:32  Ibs. 

Table  showing  the  pressure  in  Ibs.  per  square  inch  and  square  foot,  produced  by  water 
at  various  depths. 


Deplh  in 

e 

Pressure  per 
Sqoare  Inch. 

Pressure  per                Depth  iu 
Square  Foot.                    Feet. 

Pressure  per 
Square  Inch. 

Pressure  per 
Square  Foot. 

I. 
II. 
III. 

IV. 
V. 

Ibs. 
0-4323 
0-8G56 
1-2084 
1-7312 
2-1G40 

Ibs. 
62-3239 
124-6464 
186-^696 
249-2928 
811.6160 

VI. 
VII. 
VIII. 
IX. 
X. 

Ibs. 
2-5068 
3  -Of  96 
3-4624 
3-Sf'52 
4-3280 

11s. 
373-9^92 
4!  6-  6^4 
4«S-5S56 
560-0088 
62B-2320 

"  By  the  aid  of  the  above  table,  the  actual  pressure  of  water  on  each  part  of  the  surface 
of  a  vessel  containing  it  can  always  be  determined,  the  depth  of  such  part  being  given. 
Thus,  for  example,  if  it  be  required  to  know  the  pressure  upon  a  square  foot  of  the  bot- 
tom of  a  vessel  where  the  depth  of  the  water  is  25  feet,  we  find,  from  the  above  table,  that 
the  pressure  upon  a  square  foot  at  the  depth  of  2  feet  is  124'64!34  Ibs.  ;  and,  consequently, 
the  pressure  at  the  depth  of  20  feet  is  1246-464  Ibs.  ;  to  this,  let  the  pressure  at  the  depth 
of  5  feet,  as  given  in  the  table,  be  added:  1246-464-(-311-516=1558-OSO  Ibs.,  which  is,  there- 
fore, the  required  pressure. 

"If  the  liquid  contained  in  the  vessel  be  not  water,  but  any  other  whose  relative  weight 
compared  with  water  is  known,  the  calculation  is  made  first  for  water,  and  the  result  being 
multiplied  by  the  number  expressing  the  proportion  of  the  weight  of  the  given  liquid  to 
that  of  water,  the  result  will  be  the  required  pressure."— Lardner. 


132 


WELLS'S  NATUKAL  PHILOSOPHY. 


How    is    the 
pressure   of  a 
liquid   exerted 
laterally  ? 


298.  As  liquids  transmit  pressure  equally  in 
all  directions,  this  pressure  will  act  sideways 
as  well  a*s  downward,  and  the  pressure  at  any 
115.  point  upon  the  side  of  a  vessel  con- 

taining a  liquid,  will  be  in  propor- 
tion to  the  perpendicular  depth  of 
that  point  below  the  surface. 

Fig.  115  represents  a  vessel  of  water  with 
orifices  at  the  side,  at  different  distances  from 
the  surface.  The  water  will  flow  out  with  a 
force  proportionate  to  the  pressure  of  the  water 
at  these  several  points,  and  this  pressure  is 
proportionate  to  the  depth  below  the  surface. 
Thus,  at  a  the  water  will  flow  out  with  the 
least  force,  because  the  pressure  is  least  at  that 

point.     At  b  and  c  the  force  and  pressure  will  be  greater,  because  they  are 

situated  at  a  greater  depth  below  the  surface. 

299.  To  find  the  pressure  upon  the  side  of  a 
vessel  containing  water,  multiply  the  area  of 
the  side  by  one  half  its  whole  depth  below  the 
surface,  and  this  product  again  by  the  weight 
of  a  cubic  foot  of  water. 

Suppose  A  C,  Fig.  116,  to  represent  the  section  of  the 
side  of  a  canal,  or  a  vessel  filled  with  water,  and  let  the 
whole  depth,  A  C,  be  10  feet:  then  at  the  middle  point, 
B,  the  depth,  A  B,  will  be  5  feet.  Now  the  pressure  at 
C  is  produced  by  a  column  of  water  whose  depth  is  10 
feet,  but  the  pressure  at  B  is  produced  by  a  column 
whose  depth  is  5  feet,  which  is  the  average  between  the 
pressure  at  the  surface  and  at  the  bottom,  or  the  average  of  the  entire  pressure 
upon  the  side.  Hence  the  total  pressure  upon  the  side  of  a  vessel  containing 
water  will  be  equal  to  the  weight  of  a  column  of  water  whose  base  is  equal  to 
the  area  of  that  side,  and  whose  height  is  equal  to  one  half  the  depth  of  the 
liquid  in  the  vessel,  or,  in  other  words,  to  the  depth  of  the  middle  point  of  the 
side  below  the  surface. 

As  the  pressure  upon  the  sides  of  a  reservoir  containing  wa- 
ter increases  with  the  depth,  the  walls  of  embankments,  dams, 
canals,  etc.,  are  made  broader  or  thicker  at  the  bottom  than 
at  the  top  (as  in  Fig.  114).  For  the  same  reason,  in  order  to 
render  a  cistern  equally  strong  throughout,  more  hoops  should 
be  placed  near  the  bottom  than  at  the  top. 

If  a  surface  equal  to  the  side  of  a  vessel  containing  liquid  were  laid  upon 
the  bottom,  then  the  pressure  upon  the  surface  would  be  double  the  actual 


How  may  the 
pressure  upon 
the  side  of  a 
vessel  of  water 
be  calculated  ? 


FIG.  116. 


"Why  should 
an  embankment 
be  made  strong- 
er at  the  bot- 
tom than  at  the 
top? 


HYDROSTATICS.  133 

pressure  on  the  side;  for  in  this  instance  the  surface  sustains  the  weight  of  a 
column  equal  in  height  to  the  whole  depth,  while  the  column  of  pressure  upon 
the  side  is  only  equivalent  to  one  half  the  depth. 

HOW  does  the         300.  The  actual   pressure  produced  upon 

g[venUquuntity  *ne  bottom  and  sides  of  a  vessel  which  con- 

S£reqwuhc°tts  tams   a   liquid,    is   always  greater    than  the 

weight?  weight  of  the  liquid. 

In  a  cubical  vessel,  for  example,  the  pressure  upon  the  bottom  will  be 
«qual  to  the  weight  of  the  liquid,  and  the  pressure  on  each  of  the  four  sides 
will  be  equal  to  one  half  the  weight ;  consequently  the  whole  pressure  on  the 
bottom  and  sides  will  be  equal  to  three  times  the  weight  of  the  liquid. 

inwhatcondi-         301.  The  surface  of  a  liquid  when  at  rest  is 

tion  is  the  sur-  TT  T 

face  of  a  liquid    always  HORIZONTAL,  or  LEVEL. 

The  particles  of  a  liquid  having  perfect  freedom  of  motion 
surface  rf  a*  i£  among  themselves,  and  all  being  equally  attracted  by  gravita- 
qui<?  at  rest  tion,  the  whole  body  of  liquid  will  tend  to  arrange  itself  in 
level  ?  such  a  manner  that  all  the  parts  of  its  surface  shall  be  equally 

distant  from  the  earth's  center,  which  is  the  center  of  attraction. 

A  perfectly  level  surface  really  means  one  in  which  every 
true* definition  part  of  the  surface  .is  equally  near  the  center  of  the  earth;  it 
surface?eri0al  must  be»  therefore>  in  fact>  a  spherical  surface.  But  so  large 
is  the  sphere  of  which  such  a  surface  forms  a  part,  that  in 
reservoirs  and  receptacles  of  water  of  limited  extent,  its  sphericity  can  not  be 
noticed,  and  it  may  be  considered  as  a  perfect  plane  and  level ;  but  when  the 
.surface  of  water  is  of  great  extent,  as  in  the  case  of  the  ocean,  it  exhibits  this 
rounded  form,  conforming  to  the  figure  of  the  earth,  most  perfectly.*  This 
sphericity  of  the  surface  of  the  ocean  is  illustrated  by  the  fact,  that  the  masts, 
of  a  ship  appproaching  us  at  sea,  are  visible  long  before  the  hull  of  the 
FlQ  jjij  vessel  can  be  seen.  In  Fig. 

117  only  that  part  of  the 
ship  above  the  line  A  C  can 
be  seen  by  the  spectator  at 
A,  because  the  rest  of  the 
vessel  is  hidden  by  the  swell 
of  the  curve  of  the  surface  of  the  ocean,  or  rather  of  the  earth,  D  E. 

in  what  man.  302.  Water,  or  other  liquids  will  always  rise 
Sw  SI1  ainiqa  to  an  exact  level  in  any  series  of  different 
oi-vesseL^om-  tubes,  pipes,  or  other  vessels  communicating 
with  each  other. 


*  A  hoop  surrounding  the  earth  would  bend  from  a  perfectly  straight  line  eight  inches 
in  a  mile.  Consequently,  if  a  segment  of  the  surface  of  the  earth,  a  mile  long  were 
cut  off,  and  laid  on  a  perfect  plane,  the  center  of  the  segment  would  be  only  four  inches 
higher  than  the  edges.  A  small  portion  of  it,  therefore,  for  all  ordinary  purposes,  may 
te  considered  as  a  perfect  plane. 


134 


WELLS'S  NATURAL  PHILOSOPHY. 


FIG.  118. 


On  what  prin- 
ciple are  ire 
enabled  to  con- 
vey water  in 
aqueducts  over 
uneven  sur- 
faces? 


This  fact  is  sufficiently  illustrated 
by  reference  to  Fig.  118. 

303.  It  is  upon 

the  application  of 

the  principle  that 

water  in  pipes  will 

always  rise  to  the 
height,  or  level  of  its  source,  that  all 
arrangements  for  conveying  water 
over  uneven  surfaces  in  aqueducts,  or  closed  pipes  depend.  The  water 
brought  from  any  reservoir  or  source  of  supply,  in  or  near  a  town  or  building, 
may  be  delivered  by  the  effect  of  gravity  alone  to  every  location  beneath  the 
level  of  the  reservoir;  the  result  not  being  affected  by  the  inequalities  of  the 
surface  over  which  the  water  pipes  may  pass  in  their  connection  between  the 
reservoir  and  the  point  of  delivery.  So  long  as  they  do  not  rise  above  the 
level  of  the  source  of  supply,  so  long  will  the  water  continue  to  flow. 

Fig.  119  represents  the  line  of  a  modern  aqueduct: — a  a  a  represents  the 
water  level  of  a  pond  or  reservoir  upon  elevated  ground.  From  this  pond  a 
line  of  pipe  is  laid,  passing  over  a  bridge  or  viaduct  at  d,  and  under  a  river  at 
c.  The  fountains  at  b  &,  show  the  stream  rising  to  the  level  of  its  source  in 
the  pond  a,  at  two  points  of  very  different  elevation. 

FIG.  119. 


The  ancients,  in  constructing  aqueducts,  do  not  seem  to  have  ever  practi- 
cally applied  this  principle,  that  water  in  pipes  rises  to  the  level  of  its  source. 
"When,  in  conducting  water  from  a  distant  source  to  supply  a  city,  it  became 
necessary  to  cross  a  ravine  or  valley,  immense  bridges,  or  arches  of  masonry 
were  built  across  it,  with  great  labor  and  at  enormous  expense,  in  order  that 
the  water-flow  might  be  continued  nearly  horizontally.  At  the  present  day, 
the  same  object  is  effected  more  perfectly  by  means  of  a  simple  iron  pipe, 
bending  in  conformity  with  the  inequalities  of  surface  over  which  it  passes. 

In  the  construction  of  pipes  for  conveying  water,  it  is  neces- 
sary that  those  parts  which  are  much  below  the  level  of  the 
reservoir,  should  have  a  great  degree  of  strength,  since  they 
sustain  the  bursting  pressure  of  a  column  of  water  whose 
height  is  equal  to  the  difference  of  level.  A  pipe  with  » 
diameter  of  4  inches,  150  feet  below  the  level  of  a  reservoir,  should  have  suf- 


In  what  man- 
ner should 
pipes  for  the 
conveyance  of 
water  be  con- 
structed ? 


HYDROSTATICS. 


135 


FIG.  120. 


ficient  strength  to  bear  with  security  a  bursting  pressure  of  nearly  5  tons  for 
each  foot  of  its  length. 

Upon  the  principle  that  water  tends  to  rise  to  the  level  of  its  source,  orna- 
mental fountains  may  be  constructed.  Let  water  spout  upward  through  a  pipe 
communicating  with  the  bottom  of  a  deep  vessel,  and 
it  will  rise  nearly  to  the  height  of  the  upper  sur- 
face of  the  water  in  the  vessel.  The  resistance  of 
the  air,  and  the  falling  dropsr  prevent  it  from  rising 
to  the  exact  level.  Let  A,  Fig.  120,  represent  a 
cistern  filled  with  water  to  a  constant  height,  B. 
If  four  bent  pipes  be  inserted  in  the  side  of  the 
cistern  at  different  distances  below  the  surface,  the 
water  will  jet  upward  from  all  the  orifices  to  nearly 
the  same  level. 

The  phenomena  of  Artesian  Wells,  and  the  plan 
of  boring  for  water,  depend  on  the  same  principle. 

304.  An  ARTESIAN  WELL  is  a  cylindrical 
excavation  formed  by  boring  into  the  earth 
with  a  species  of  auger,  until  a  sheet  or  vein  of  water  is 
found,  when  the  water  rises  through  the  excavation.  Such 
excavations  are  called  Artesian,  because  this  method  was 
employed  for  obtaining  water  at  Artois  in  France. 

The  reason  that  the  water  rises  in  Artesian,  and  sometimes 
Why  does  the 
water  rise  in      in  ordinary  wells,  to  tne  surface,  is  as  follows :    The  surface 

of  the  &lobe  [s  formed  of  different  layers,  or  strata,  of  different 
materials,  such  as  sand,  gravel,  clay,  stone,  etc.,  placed  one 
upon  the  other.  In  particular  situations,  these  strata  do  not  rest  horizontally 
upon  one  another,  but  are  inclined,  the  different  strata  being  like  cups,  or 
basins  placed  one  within  the  other,  as  in  Fig.  121.  Some  of  these  strata  are 
composed  of  materials,  as  sand  or  gravel,  through  which  water  will  soak  most 

readily;  while  other  strata, 
like  clay  and  rock,  will  not 
allow  the  water  to  pass 
through  them.  If,  now, 
we  suppose  a  stratum  like 
sand,  pervious  to  water,  to 
be  included  as  at  a  a,  Fig. 
121,  between  two  other 
strata  of  clay  or  rock,  the 
water  falling  tipon  the  un- 
covered margin  of  the  sandy  stratum  a  a,  will  be  absorbed,  and  penetrate  through 
its  whole  depth.  It  will  be  prevented  from  rising  to  the  surface  by  the  im- 
pervious stratum  above  it,  and  from  sinking  lower,  by  the  equally  impervious 
stratum  below  it.  It  will,  therefore,  accumulate  as  in  a  reservoir.  If,  now,  we 


Fiu.  121. 


136  WELLS'S  NATURAL  PHILOSOPHY. 

bore  down  through  the  upper  stratum,  as  at  &,  until  we  reach  the  stratum 
containing  the  water,  the  water  will  rise  in  the  excavation  to  a  certain  height, 
proportional  to  the  height  or  level  of  the  water  accumulated  in  the  reser- 
voir a  a  from  which  it  flows.* 

What  is  the  ori-  3°5'  The  rain  which  falls  uPon  the  surface  of  the  earth 
gin  of  springs?  sinks  downward  through  the  sandy  and  porous  soil,  un- 
til a  bed  of  clay  or  rock,  through  which  the  water  can  not 
penetrate,  is  reached.  Here  it  accumulates,  or  running  along  the  surface 
of  the  impervious  stratum,  bursts  out  in  some  lower  situation,  or  at  some  point 
where  the  impervious  bed  or  stratum  comes  to  the  surface  in  consequence  of  a 

valley,  or  some  depression. 

' IG*  Such  a  flow  ef  water  consti- 

tutes a  spring.  Suppose  a, 
Fig.  122,  to  be  a  gravel  hill, 
and  b  a  stratum  of  clay  or 
rock,  impervious  to  water. 
The  fluid  percolating  through 
the  gravel  would  reach  the 

impervious  stratum,  along  which  it  would  run  until  it  found  an  outlet  at  c,  at 
the  foot  of  the  hill,  where  a  spring  would  be  formed. 

306.  If  there  are  no  irregularities  in  the  surface,  so  situated 
Why  does  water 

collect  in  an  or-  as  to  allow  a  spring  to  burst  forth,  or  if  a  spring  issues  out 
dinary  well  ?  a^  some  point  of  the  porous  earth  considerably  above  the  sur- 
face of  the  clay,  or  rock,  upon  which  at  some  depth  all  such  earth  rests,  the 
water  soaking  downward  will  not  all  be  drained  off,  but  will  accumulate,  and 
rise  among  the  particles  of  soil,  as  it  would  among  shot,  or  bullets,  in  a  water- 
tight vessel  If  a  hole,  or  pit,  be  dug  into  such  earth,  reaching  below  the 
level  of  the  water  accumulated  in  it,  it  will  soon  be  filled  up  with  water  to 
this  level,  and  will  constitute  a  well  The  reason  why  some  wells  are  deeper 
than  others,  is,  that  the  distance  of  the  impervious  stratum  of  clay  below  the 
surface  is  different  in  different  localities. 

307.  All  wells  and  springs,  therefore,  are  merely  the  rain- 
source  do  all     water  which  has  sunk  into  the  earth,  appearing  again,  and 
Things*  derive      gradually  accumulating,  or  escaping  at  a  lower  level. 

their  water  ?  308.  The  property  of  liquids  to  assume  a  horizontal  sur- 

What  is  a  ^ace  ™  practically  taken  advantage  of  in  ascertaining  whether 
Water,  or  a  surface  [s  perfectly  horizontal,  or  level,  and  is  accomplished 
by  means  of  an  instrument  known  as  the  "  WATER"  or 
"SPIRIT  LEVEL."  This  consists  of  a  small  glass  tube,  b  c,  Fig.  123,  filled 
with  spirit,  or  water,  except  a  small  space  occupied  with  air,  and  called 

*  In  the  great  Artesian  wells  of  Grenelle,  near  Paris,  and  of  Kissingen,  in  Bavaria,  tha 
water  risps  from  depths  of  1,800  and  1,900  feet  to  a  considerable  height  above  the  surface 
of  the  earth.  The  well  of  Paris  is  capable  of  supplying  water  at  the  rate  of  14  millions 
of  gallons  per  day.  The  region  of  country  in  which  this  water  fell,  from  the  curvature 
of  the  layers,  or  strata  of  material  through  which  the  excavation  was  made,  mast  hav» 
been  distant  two  hundred  miles  or  more. 


HYDROSTATICS. 


187 


FIG.  123. 


\ 


the  air-bubble,,  a.  In  whatever  position  the  tube 
may  be  placed,  the  bubble  of  air  will  rest  at  the  high- 
est point.  If  the  two  ends  of  the  tube  are  level,  or 
perfectly  horizontal,  the  air-bubble  will  remain  in 
the  center  of  the  tube  ;  but  if  the  tube  inclines  ever 
so  little,  the  bubble  rises  to  the  higher  end.  For  practical  use  the  glass-tube 
is  inclosed  in  a  wood,  or  brass  case,  or  box. 

.  309.  The  method  of  conducting  a  canal  through  a  country, 

cipie  are  canals  the  surface  of  which  is  not  perfectly  horizontal,  or  level,  de- 
Derated?  d  a"d  pends  upon  this  same  Pr°Perty  of  liquids.  In  order  that  boats 
may  sail  with  ease  in  both  directions  of  the  canal,  it  is  neces- 
sary that  the  surface  of  the  water  should  be  level.  If  one  end  of  a  canal 
were  higher  than  the  other,  the  water  would  run  toward  the  lower  extremity, 
overflow  the  banks,  and  leave  the  other  end  dry.  But  a  canal  rarely, 
if  ever,  passes  through  a  section  of  country  of  any  great  extent,  which  is 
not  inclined,  or  irregular  in  its  surface.  By  means,  however,  of  expedients 
called  LOCKS,  a  canal  can  be  conducted  along  any  declivity.  In  the  forma- 
tion of  a  canal,  its  course  is  divided  into  a  series  of  levels  corresponding 
with  the  inequalities  of  the  surface  of  the  country  through  which  it  passes. 
These  levels  communicate  with  each  other  by  locks,  by  means  of  which 
boats  passing  in  any  direction  can  be  elevated,  or  lowered  with  ease,  rapidity; 
and  safety. 

FIG.  124.  Fig.  124  represents  a  section  of 

a  lock,  and  Fig.  125  the  construc- 
tion of  the  LOCK  GATES.  The  sec- 
tion of  Fig.  125  represents  a  place 
where  there  is  a  sudden  fall  of  the 
ground,  along  which  the  canal  has 
to  pass.  A  B  and  C  D  are  two 
gates  which  completely  intercept 

the  course  of  the  water,  but  at  the  same  time  admit  of  being  opened  and 
closed.  A  H  is  the  level  of  the  water  in  that  part  of  the  canal  lying 
above  the  gate  A  B,  and  E  F  and  F  G  the  levels  below  the  gate  A  B.  The 
part  of  the  canal  included  between  two  gates,  as  E  F,  is  called  a  lock,  because 
when  a  vessel  is  let  into  it,  it  can  be  shut  by  closing  both  pair  of  gates.  If 
now  it  is  required  to  let  a  boat  down  from  the  higher  level,  A  H,  to  the  lower 
level,  E  G,  the  gates  G  D  are  closed  tightly,  and  an  opening  made  in  the 
gates  A  B  (shown  in  Fig.  125),  which  allows  the  water  to  flow  gradually  from 
A  H  into  the  lock  A  E  F  G,  until  it  attains  a  common  level,  HAG.  The 
gate  A  B  is  then  opened,  and  the  boat  floats  into  the  lock  A  B  C  D.  The 
gates  A  B  are  then  closed,  and  an  opening  made  in  gates  C  D,  which  allows 
the  water  to  flow  from  the  space  A  E  F  C,  until  it  comes  to  the  common 
level,  E  F  G.  The  gate  C  D  is  then  opened,  and  the  boat  floats  out  of 
the  locks  into  the  continuation  of  the  canal.  To  enable  a  boat  to  pass  from 
the  lower  level,  E  F  G,  to  the  superior  level,  A  H,  the  process  here  described 
is  reversed.  .  > 


138 


WELLS'S   NATURAL   PHILOSOPHY. 
FiG.  125. 


With    what 
force  is  a  float- 
ing body  press- 
ed upward  ? 

How    much 
water    will    a 
solid  immersed 
ia  it  displace? 


What  is  Buoy- 
ancy ? 


310.  When  a  solid  is  immersed  in  a  liquid 
it  will  be  pressed  upward  with  a  force  equal 
to  the  weight  of  the  liquid  it  displaces. 

311.  A  solid  immersed  in  water  will  displace 
as  much  of  the  liquid  as  is  equal  in  volume  to 
the  part  immersed. 

312.  BUOYANCY  is  the  name  applied  to  the 
force  by  which  a  solid  immersed  in  a  liquid  is 

heaved,  or  pressed  upward. 

The  resistance  offered  when  we  attempt  to  sink  a  body  lighter  than  water 
in  that  liquid,  proves  that  the  water  presses  with  a  force  upward  as  well  as 
downward.  Upon  this  fact  the  laws  of  floating  bodies  depend  ;  and  for  this 
reason  the  bottoms  of  large  ships  aro  constructed  with  a  great  degree  of 
strength. 

313.  A  body  floating  upon  a  liquid  is  main- 
tained in  EQUILIBRIO  by  the  operation  of  grav- 
ity drawing  the  mass  downward,  and  by  the 
pressure  of  the  particles  of  the  liquid  upon 

which  it  rests,  pressing  it  upward. 

314.  In  order  that  a  body  may  float  with  sta- 
?  ^  ™  necessary  that  its  center  of  gravity 

should  be  situated  as  low  as  possible. 


How  is  a  body 
floating  upoTia 
liquid  main- 
tained in  equi- 
librio? 


tial  to  the  sta- 
bility of  a  float- 
ing body  ? 


HYDROSTATICS. 


139 


When  is  a 
floating  body 
in  its  most  sta- 
ble position  ? 


When  will  a 
solid  float,  and 
•when  sink  ? 


FIG.  126. 


What   is   the          For  this  reason,  all  vessels  which  are  light  in  proportion  to 

use  of  ballast     thejr  ^^  require  to  be  ballasted  by  depositing  in  the  lowest 
portions  of  the  vessel,  immediately  above  the  keel,  a  quantity 

of  heavy  matter,  usually  iron  or  stone.     The  center  of  gravity  may  thus  be 

brought  so  low  that  no  force  of  the  wind  striking  the  vessel  sideways  can 

capsize  it.     By  raising  the  center  of  gravity,  as  when  men  in  a  boat  stand 

upright,  the  equilibrium  is  rendered  unstable. 

A  body  floating  is  most  stable  when  it  floats  upon  its  great- 
est surface :  thus  a  plank  floats  with  the  greatest  stability 
when  placed  flat  upon  the  water;  and  its  position  is  unstable 
when  it  is  made  to  float  edgewise. 

A  solid  can  never  float  that  is  heavier,  bulk  for  bulk,  than 
the  liquid  in  which  it  is  immersed. 

If  the  weight  of  a  solid  be  exactly  equal  to  the  weight  of 

an  equal  bulk  of  liquid,  it  will  sink  in  it  until  it  is  entirely  immersed ;  but 

when  once  it  is  entirely  immersed,  then,  the  upward  and  downward  pressure 

being  equal,  the  solid  will  neither  sink  or  rise,  but  will  remain  suspended 

at  any  depth  at  which  it  may  be  placed. 

Let  A  B,  Fig.  126,  be  a  cube  of  wood  floating  in 

water;  then  the  weight  of  the  water  displaced,  or 

the  weight  of  a  volume  of  water  equal  to  A  B,  is 

equal  to  the  whole  weight  of  the  wood ;  since  the 

upward  pressure  on  the  bottom  of  A  B  is  the  same 

as  that  which  would  support  a  portion  of  water 

equal  in  bulk  to  the  displaced  water,  or  to  the  cube 

A  B ;  and  as  the  downward  pressure  of  the  body 

is  equal  to  the  upward  pressure  of  the  liquid,  it  fol- 
lows that  the  weight  of  the  cube  is  equal  to  the 

weight  of  the  water  displaced.     Hence  A  B  will 

neither  sink  or  rise. 

A  mass  of  stone,  or  any  other  heavy  substance  .< 

beneath  the  surface  of  water  is  more  easily  moved  *j 

than  upon  the  land  because,  when  immersed  in  the 

water,  it  is  lighter  by  the  weight  of  its  own  bulk  of 

water  than  it  would  be  on  land.     A  boy  will  often  wonder  why  he  can  lift  a 

stone  of  a  certain  weight  to  the  surface  of  water,  but  can  carry  it  no  farther. 

The  least  force  will  lift  a  bucket  immersed  in  water  to  the  surface ;  but  if  it 

be  lifted  farther,  its  weight  is  felt  just  in  proportion  to  the  part  of  it  which  is 

above  the  surface. 

The  weight  of  the  human  body  does  not  differ  much  from  the  weight  of  ita 

own  bulk  of  water;   consequently,  when  bathers  walk  in  water  chin-deep, 

their  feet  scarcely  press  upon  the  bottom,  and  they  have  not  sufficient  hold 

upon  the  ground  to  give  them  stability ;  a  current,  therefore,  will  easily  take 

them  off  their  feet 

The  facility  with  which  different  persons  are  able  to  float  or  swim,  depends 

upon  the  physical  constitution  of  the  body.     Corpulent  people  are  lighter, 


140  WELLS'S.  NATURAL   PHILOSOPHY. 

-bulk  for  bulk,  than  those  of  sparer  habits :  and  as  fat  possesses  a  less  specific 
gravity  than  water,  a  fat  person  will  swim  or  float  easier  than  a  thin  one. 

315.  It  is  not,  however,  necessary,  in  order  that  a  body  should  float  upon  a 
liquid,  that  the  materials  of  which  it  is  composed  should  be  specifically  lighter 
than  the  liquid.  If  the  entire  mass  of  a  solid  is  lighter  than  an  equal  volume 
.of  the  liquid,  it  will  float. 

A  thick  piece  of  iron,  weighing  half  an  ounce,  loses  in  water  nearly  one 
eighth  of  its  weight ;  but  if  it  is  hammered  into  a  plate  or  vessel,  of  such  a 
form  that  it  occupies  eight  times  as  much  space  as  before,  it  will  then  weigh 
less  than  an  equal  bulk  of  water,  and  will  consequently  float,  sinking  just  to 
the  brim.  If  made  twice  as  large,  it  will  displace  one  ounce  of  water,  conse- 
quently, twice  its  own  weight;  it  will  then  sink  to  the  middle,  and  can  be 
loaded  with  half  an  ounce  weight  before  sinking  entirely. 

HOW  can  a  316.  A  body  composed  of  any  material,  how- 
timn  an  TqTai  ever  heavy,  can  be  made  to  float  on  any  liquid, 
£elkmadeatto  however  light,  by  giving  it  such  a  shape  as 
will  render  its  bulk  or  volume  lighter  than  an 
equal  bulk  of  water. 

Iron  ships  and  boats  are  illustrations  of  this  principle.  A  ship  carrying  a 
thousand  tons'  weight  will  displace  just  as  much  water,  or  float  to  the  same 
depth,  whether  her  cargo  be  feathers,  cotton,  or  iron.  A  ship  made  of  iron 
floats  just  as  high  out  of  water  as  a  ship  of  similar  form  and  size  made  of 
wood,  provided  that  the  iron  be  proportionally  thinner  than  the  wood,  and 
therefore  not  heavier  on  the  whole. 

The  buoyancy  of  hollow  solids  is  frequently  used  for  lifting  or  supporting 
heavy  weights  in  water.  Life-preservers,  which  are  inflated  bags  of  India- 
rubber,  are  an  example.  Hollow  boxes,  or  tanks,  are  used  for  the  purpose 
of  raising  sunken  vessels.  These  boxes  are  sunk,  filled  with  water,  and 
attached  to  the  side  of  the  vessel  to  be  raised.  The  water,  by  a  connection 
of  pipes,  is  then  pumped  out  of  them,  when  the  upward  pressure  of  the  liquid 
becoming  greater  than  the  gravity  or  weight  of  the  entire  mass,  the  whole 
will  rise  and  float. 

&17.  The  buoyancy  of  liquids  is  in  propor- 
to  their  density  or  specific  gravity,  or,  in 
other  words,  a  solid  is  buoyant  in  a  liquid,  in 
proportion  as  it  is  light,  and  the  liquid  heavy. 

Thus  quicksilver,  the  heaviest,  or  most  dense  fluid  known,  supports  iron 
upon  its  surface;  and  a  man  might  float  upon  mercury  as  easily  as  a  cork 
floats  upon  water.  Many  varieties  of  wood  which  will  sink  in  oil,  float 
readily  upon  water. 

318.  The  principle  that  the  buoyancy  of  liquids  varies  in  proportion  as  their 
specific  gravity  varies,  furnishes  a  very  ready  method  of  determining  the  spe- 
cific gravity  of  a  liquid.  This  is  done  by  means  of  an  instrument  called  the 
hydrometer. 


HYDROSTATICS. 


141 


What  is  a  Hy- 
drometer ? 


How  may  the 
specific  grav- 
ity of  a  liquid 
be  determined 
by  the  Hy- 
drometer ? 


319.  The  HYDROMETER  con-  Fia.  127. 
sists  of  a  hollow  glass  tube, 
on  the  lower  part  of  which  a  spaerical 
bulb  is  blown,  the  latter  being  filled  with 
a  suitable  quantity  of  small  shot,  or 
quicksilver,  in  order  to  cause  it  to  float, 
in  a  vertical  position.  The  upper  part 
of  the  tube  contains  a  scale  graduated 
into  suitable  divisions.  (See  Fig.  127.) 

It  is  obvious  that  the  hydrometer 

will  sink  to  a  greater  or  less  depth  in 

different  liquids  ;  deeper  in  the  lighter 

ones,  or  those  of  small  specific  gravity, 

and  not  so  deep  in  those  which  are 
denser,  oY  which  have  great  specific  gravity.  The 
specific  gravity  of  a  liquid  may,  therefore,  be  estimated  by  the  number  of  di- 
visions on  the  scale  which  remain  above  the  surface  of  the  liquid.  Tables 
are  constructed,  so  that,  by  their  aid,  when  the  number  on  the  scale  at  which 
the  hydrometer  floats  in  a  given  liquid  is  determined  by  experiment,  the  spe- 
cific gravity  is  expressed  by  figures  in  a  column  directly  opposite  that  number 
in -the  table. 

There  are  various  forms  of  the  hydrometer  especially  adapted  for  determin- 
ing the  density,  or  specific  gravity,  of  spirits,  oils,  syrups,  lye,  etc.  It  affords 
a  ready  method  of  determining  the  purity  of  a  liquid,  as,  for  instance,  alco- 
hol The  addition  of  water  to  alcohol  adds  to  its  density,  and  therefore  in- 
creases its  buoyancy.  The  addition  of  water,  therefore,  will  at  once  be  shown 
by  the  less  depth  to  which  the  hydrometer  will  sink  in  the  liquid.  The 
adulteration  of  sperm  oil  with  whale,  or  other  cheaper  oils,  may  be  shown  in 
the  same  manner. 

320.  For  the  reason  that  the  buoyancy  of  a  liquid  is  proportioned  to  its 
density,  a  ship  will  draw  less  water,  or  sail  lighter  by  one  thirty-fifth  in  the 
heavy  salt  water  of  the  ocean,  than  in  the  fresh  water  of  a  river;  for  the 
same  reason  it  is  easier  to  swim  in  salt  than  in  fresh  water.* 

*  "A  floating  bod  sirks  to  the  same  depth  whether  the  mass  of  liquid  supporting  it 
be  great  or  small,  as  is  seen  when  an  earthen  cup  is  placed  first  in  a  pond,  and  then  in  a 
second  cup  only  so  much  larger  than  itself,  that  a  very  small  quantity  of  water  will  suffice 
to  fill  up  the  interval  between  them.  An  ounce  of  water  in  this  way  may  be  made  to  float 
substances  of  much  greater  weight.  And  if  a  large  ship  were  received  into  a  dock,  or 
case,  so  exactly  filling  it  that  there  were  only  half  an  inch  of  interval  between  it  and  the 
wall,  or  side  of  the  containing  space,  it  would  float  as  completely  when  the  few  hogsheads 
of  water  required  to  fill  this  little  interval  up  to  its  usual  water-mark  were  poured  in,  as 
if  it  were  on  the  high  seas.  In  some  canal  locks,  the  boats  just  fit  the  place  in  which  they 
have  to  rise  and  fall,  and  thus  diminish  the  quantity  of  water  necessary  to  supply  th« 
lock."— ArnotL 


142 


WELLS'S   NATURAL   PHILOSOPHY. 


Explain  the 
phenomena  ob- 
served when 
the  hand  is 

§  lunged      into 
ifferent      liq- 
uids. 


SECTION    I  . 

CAPILLARY     ATTRACTION. 

321.  If  we  plunge  the  hand  into  a  vessel  of  water,  and 
withdraw  it,  it  is  said  to  be  wet ;  that  is,  it  is  covered  with  a 
thin  film,  or  coating  of  water,  which  adheres  to  it,  in  opposi- 
tion to  the  tendency  of  the  attraction  of  gravitation  to  make 
it  fall  off.     There  is,  therefore,  an  attraction  between  the  par- 
ticles of  the  water  and  the  hand,  which,  to  a  certain  extent, 

is  stronger  than  the  influence  of  gravitation. 

If  now  we  plunge  the  hand  into  a  vessel  of  quicksilver,  no  adhesion  of  the 
particles  of  the  mercury  to  the  hand  will  take  place,  and  the  hand,  when 
withdrawn,  will  be  perfectly  dry. 

If  we  plunge  a  plate  of  gold,  however,  into  water  and  quicksilver,  it  will 
be  wet  equally  by  both,  and  will  come  out  of  the  quicksilver  covered  with  a 
White  coating  of  that  liquid. 

It  is,  therefore,  obvious  that  a  certain  molecular  attraction  exists  between 
certain  liquids  and  certain  solids,  which  does  not  prevail  to  the  same  extent 
between  others. 

322.  That  variety  of  molecular  force  which 
manifests  itself  between  the  surfaces  of  solids 
and  liquids  is  called  CAPILLARY  ATTRACTION. 

This  name  originates  from  the  circumstance,  that  this  class 
of  phenomena  was  first  observed  in  small  glass  tubes,  the 
bore  of  which  was  not  thicker  than  a  hair,  and  which  were 
hence  called  Capittary  Tubes,  from  the  Latin  word  capillus,  which  signifies  a  hair. 
323.  If  we  take  a  series  of  glass  tubes  of  very  fine  bore, 
piliary  Attrac-      but  of  different  diameters,  and  place  them  in  a  vessel  of  water, 
which  has  been  colored  in  order  to  show  the  effect  more  strik- 
ingly, we  shall  see  that  the  water  will  rise  in  the  tubes  to 
various  heights,  attaining  the  greatest  degree  of  elevation  in  the  smallest  tube. 
128.  The  height  at  which  the  same  liquid  will  rise  in 

any  given  tube  is  always  uniform,  but  it  varies  for 
different  liquids. 

Fig.  128  is  an  enlarged  representation  of  the 
manner  in  which  water  will  rise  in  tubes  of  differ- 
ent diameters. 

The  simplest  method  of  exhibiting  capillary  at- 
traction is  to  immerse  the  end  of  a  piece  of  ther- 
mometer tube  in  water  (see  Fig.  129)  which  has 
been  tinted  with  ink.  The  liquid  will  be  seen  to 
ascend,  and  will  remain  elevated  in  the  tube  at  a 
considerable  height  above  the  surface  of  the  liquid 
in  the  vessel 

The  ordinary  definition  of  capillary  attraction  is,  that  form  of  attraction  which 


What  is  Ca- 
pillary Attrac- 
tion? 


What  is  the 
origin  of  the 
term? 


ry  . 

tion  be    illus- 
trated ? 


CAPILLARY    ATTRACTION. 


143 


What  will  be 
the  condition 
of  the  surface 
of  a  liquid 
which  wots  the 
sides  of  the  ves- 
sel coutainiag 
it? 

When  the  liq- 
uid does  not 
wet  the  sides 
of  the  vessel, 
what  will  be 
the  condition 
of  its  surface  ? 

FIG.  130. 


causes  liquids  to  ascend  above  their  level  in  capillary  tubes.        FIG.  129. 
It,  however,  is  not  strictly  correct,  as  this  force  not  only  acts 
in  elevating   but  in   depressing  liquids  in  tubes,  and  is  at 
work  wherever  liquids  are  in  connection  with  solid  bodies. 

324.  If  a  liquid  be  poured  into  a  vessel,  as 
water  in  glass,  whose  sides  are  of  such  a  nature 
as  to  be  wetted  by  it,  the  liquid  will  be  elevated 
above  the  general  level  of  its  surface  at  the 
points  where  it  touches  the  sides  of  the  ves- 
sel. This  is  shown  in  Fig.  130. 

If,   however,  the  liquid  is  poured  into  a 
vessel  whose  sides  are  of  such  a  nature  that 
they  are  not  wetted  by  it,  as  in  the  case  of 
quicksilver  in  a  glass  vessel,  then  the  liquid 
will  be  depressed  below  the  general  level  of  its  surface  at  tho 
points  where  it  comes  in  con- 
tact  with  the  sides  of  the  ves- 
sel.   This  is  shown  in  Fig.  131. 
325.  If  two  plates  of  glass, 
A  and  B,  Fig.  132,  be  plunged 
into  water  at  their  lower  ex- 
tremities, with  their  faces  ver- 
tical and  parallel,  and  at  a  cer- 
tain distance  asunder,  the  water  will  rise  at  the  points  m  and  «,  where  it  is  in 

contact  with  the  glass;  but  at 

FlG-  132-  all  intermediate  points,  beyond 

a  small  distance  from  the  plates, 
the  general  level  of  the  surfaces 
E,  C,  and  D,  will  correspond. 

If  the  two  plates,  A  and  B, 
are  brought  near  to  each  other, 
as  in  Fig.  133,  the  two  curves, 
~—  m  and  n,  will  unite,  so  as  to  form 

a  concave  surface,  and  the  water 
at  the  same  time  between  them  will  be  raised  above  the  general  level,  E  and 


D,  of  the  water  in  the  vessel.  If  the  plates 
be  brought  still  nearer  together,  as  in  Fig.  134, 
the  water  between  them  will  rise  still  higher, 
the  force  which  sustains  the  column  being  in- 
creased as  the  distance  between  the  plates  is 
diminished 

326.  The  height  to 
which  water  will  rise  in 


FIG.  133. 


To  what  is  the 
elevation  of 
water  in  capil- 
lary tubes  pro- 
portioned ? 


capillary  tubes  is  in  proportion  to  the  small- 
ness  of  their  diameters. 


144 


WELLS'S   NATURAL   PHILOSOPHY. 


FiG.  134. 


Thus  in  two  tubes,  one  of  which  is  double 
the  diameter  of  the  other,  the  fluid  will  riso 
to  twice  the  height  in  the  small  tube  that 
it  will  in  the  larger.  The  truth  of  this 
principle  can  be  made  evident  by  the  fol- 
lowing beautiful  and  simple  experiment. 
Two  square  pieces  of  plate-glass,  C  and  B, 
Fig.  135,  are  arranged  so  that  their  sur- 
faces form  a  minute  angle  at  A.  This  po- 
sition may  be  easily  given  them  by  fasten- 


FiG.   135. 


ing  with  wax  or  cement.  "When  the  ends  of 
the  plates  are  placed  in  the  water,  as  shown  in 
the  figure,  the  water  rises  in  the  space  between 
them,  forming  the  curve,  which  is  called  an 
hyperbola.  The  elevation  of  the  water  between 
the  two  surfaces  will  be  the  greatest  at  the 
points  where  the  distance  between  the  plates  is 
the  least. 

327.  The  figure  of  the  surface  which  bounds 
a  liquid  in  a  capillary  tube  will  depend  upon 
the  extent  of  the  attraction  which  exists  between 
the  particles   of  the  liquid   and  the  surface  of 
the  tube*     Thus,  a  column  of  water  contained  in  a  glass  capillary  tube  will 
have  a  concave  form  of  surface,  as  in  Fig.  136,  since  the  FJQ  13g     FIQ  13T 
attraction  of  glass  for  water  exceeds  the  attraction  of  the 
particles  of  water  for  each  other ;  a  surface  of  mercury,  on 
the  contrary,  in  a  similar  tube,  will  be  convex,  see  Fig. 
137,  since  the  attraction  of  glass  for  mercury  is  less  than 
the  mutual  attraction  of  the  particles  of  mercury. 

328.   In  a  capillary  tube   a 
liquid   will    ascend    above    its 
general  level,  when  it  wets  the 
tube  ;  and  is  depressed  below  its  level  when 
it  does  not  wet  it. 

„  329.  If  the  surface  of  a  body  repels  a  liquid,  such  a  body, 

needle  be  made  though  heavier,  bulk  for  bulk,  than  the  liquid,  may,  under 
water  ?*  UP<>a  some  circumstances,  float  upon  it ;  and  so  present  an  apparent 
exception  to  the  general  hydrostatic  law  by  which  solids 
which  are  heavier  than  liquids,  bulk  for  bulk,  will  sink  in  them.  An  exam- 
ple of  this  may  be  shown  by  slightly  greasing  a  fine  sewing-needle,  and  then  , 
placing  it  carefully  in  the  direction  of  its  length  upon  the  surface  of  water. 
The  needle,  although  heavier,  bulk  for  bulk,  than  water,  will  float. 

The  power  of  certain  insects  to  walk  upon  the  surface  of  water  without 
sinking,  has  been  explained  upon  the  same  principle.  The  feet  of  these  in- 
sects, like  the  greased  needle,  have  a  capillary  repulsion  for  the  water,  and 


When  will  a 
liquid  be  ele- 
vated and  when 
depressed  in  a 
capillary  tube  ? 


CAPILLARY   ATTRACTION. 


145 


When  will  a 
liquid  fail  to 
wet  a  solid  ? 


when  they  apply  them  to  the  surface  of  water,  instead  of  sinking  in  it,  they 
produce  depressions  upon  it. 

For  a  like  reason,  water  will  not  flow  through  a  fine  sieve,  the  wires  of 
which  have  been  greased. 

330.  A  liquid  will  not  wet  a  solid  when  the 
force  of  adhesion  developed  between  the  par- 
ticles of  the  liquid  and  the  surface  of  the  solid, 

is  less  than  half  the  cohesive  force  which  exists  between 
the  particles  of  the  liquid. 

331.  The  fact  of  the  strong  adhesion  FIG.  138. 
"Dope  Pump?"    which  exists  between  water  and  the 

fibers  of  a  rope,  has  been  taken  ad- 
vantage of  in  the  construction  of  a  kind  of  pump,  called 
the  "  Rope,"  or  "  Vera's"  Pump,  Fig.  138.  It  con- 
sists of  a  cord  passing  over  two  wheels,  a  and  6,  the 
lower  one  of  which  is  immersed  in  water.  A  rapid 
motion  is  given  to  the  wheels  by  means  of  the  crank 
d,  and  the  water,  by  adhering,  follows  the  rope  in  its 
movements,  and  is  discharged  into  a  receptacle  above. 
What  are  fa  Illustrations  of  capillary  attraction 

miiiar  iiiustra-  are  most  familiar  in  the  experience  of 
every-day  life.  The  wick  of  a  lamp, 
or  candle,  lifts  the  oil,  or  melted  grease 
which  supplies  the  flame,  from  a  surface  often  two  or 
three  inches  below  the  point  of  combustion.  In  a 
cotton-wick,  which  is  the  material  best  adapted  for  this  purpose,  the  mi- 
nute, separate  fibers  of  the  cotton  themselves  are  capillary  tubes,  and  the  in- 
terstices between  the  filaments  composing  the  wick  are  also  capillary  tubes ; 
in  these  the  oil  ascends.  The  oil,  however,  can  not  be  lifted  freely  beyond  a 
certain  height  by  capillary  attraction :  hence,  when  the  surface  of  the  oil  is 
low  in  the  lamp,  the  flame  becomes  feeble,  or  expires. 

If  the  end  of  a  towel,  or  a  mass  of  cotton  thread,  be  immersed  in  a  basin  of 
water,  and  the  remainder  allowed  to  hang  over  the  edge  of  the  basin,  the 
water  will  rise  through  the  pores  and  interstices  of  the  cloth,  and  gradually 
wet  the  whole  towel.  In  this  way  the  basin  may  be  entirely  emptied. 

If  sand,  a  lump  of  sugar,  or  a  sponge,  have  moisture  beneath  and  slightly 
in  contact  with  it,  it  will  ascend  through  the  pores  by  the  agency  of  capillary 
attraction  in  opposition  to  gravity,  and  the  entire  mass  will  become  wet. 

The  lower  story  of  a  house  is  sometimes  damp,  because  the  moisture  of  the 
ground  ascends  through  the  pores  of  the  materials  constituting  the  walls  of 
the  building.  Wood  imbibes  moisture  by  the  capillary  attraction  of  its  pores, 
and  expands  or  swells  in  consequence.  This  fact  has  been  taken  advantage 
of  for  splitting  stones ;  wedges  of  dry  wood  are  driven  into  grooves  cut  in  the 
stone,  and  on  being  moistened,  swell  with  such  irresistible  force  as  to  split 
the  block  in  a  direction  regulated  by  the  groove. 

7 


146 


WELLS'S   NATURAL   PHILOSOPHY. 


ElG.  139. 


An  immense  weight  suspended  lay  a  dry  rope,  may  be  raised  a  little  way, 
by  merely  wetting  the  rope ;  the  moisture  imbibed  by  capillary  attraction  into 
the  substance  of  the  rope  causes  it  to  swell  laterally  and  become  shorter. 

Capillary  attraction  is  also  instrumental  in  supplying  trees -and  plants  with 
moisture  through  the  agency  of  the  roots  and  underground  fibers. 

what  are  the  332.  The  terms  EZOSMOSE  and  ENDOSMOSE 
ExosZseaan°d  are  applied  to  those  currents  in  contrary  direc- 
tions which  are  established  between  two  liquids 
of  a  different  nature,  when  they  are  separated  from  each 
other  by  a  partition  composed  of  a  membrane,  or  any  porous 
substance. 

The  name  Endosmose,  derived  from  a 
Greek  word,  signifies  going  in,  and  is  ap- 
plied to  the  stronger  current;  while  the 
name  Exosmose,  signifying  going  out,  is 
applied  to  the  weaker  current. 

The  phenomena  of  Endosmose  and  Ex- 
osmose,  which  are  undoubtedly  dependent 
on  capillary  attraction,  may  be  illustrated 
by  the  following  simple  experiment : — If 
we  take  a  small  bladder,  or  any  other  mem- 
branous substance,  and  having  fastened  it 
on  a  tube  open  at  both  ends,  as  is  repre- 
sented in  Fig.  139,  fill  the  bladder  with 
alcohol,  and  immerse  it,  connected  with 
the  tube,  in  a  basin  of  water,  to  such  an 
extent  that  the  top  of  the  bladder  filled 
with  alcohol  corresponds  with  the  level 
of  the  water  in  the  vessel,  in  a  short 
time  it  will  be  observed,  that  the  liqijid 
is  rising  in  the  tube  connected  with 
the  bladder,  and  will  ultimately  reach  the 
top  and  flow  over.  This  rising  of  the  al- 
cohol in  the  tube  is  evidently  due  to  the 
circumstance  that  the  water  permeates 
through  the  bladder,  with  a  certain  de- 
gree of  force,  producing  the  phenomena 

which  we  call  endosmose,  "going  in;"  the  efiect  being  to  elevate  the  alcohol  to 
a  considerable  height  in  the  tube.  At  the  same  time,  a  certain  quantity  of 
the  alcohol  has  passed  out  through  the  pores  of  the  bladder,  and  mixed  with  the 
water  in  the  external  vessel.  This  outward  passage  of  the  alcohol  we  call 
eocosmose,  "  going  out."  A  less  quantity  of  the  alcohol  will  pass  out  of  the 
bladder  in  a  given  time  to  mingle  with  the  water,  than  of  the  water  will  pass 
in,  and  consequently  the  bladder  containing  the  alcohol  having  more  liquid 
in  it  than  at  first,  becomes  strained,  and  presses  the  liquid  up  in  the  tube. 


CAPILLARY   ATTRACTION.  147 

If  we  have  a  box  divided  by  a  partition  of  porous  clay,  or  any  other  sub- 
stance of  like  nature,  and  place  a  quantity  of  syrup  on  one  side,  and  water  on 
the  other,  or  any  other  two  liquids  of  different  densities  which  freely  mix  with 
one  another,  currents  will  be  established  between  the  two  in  opposite  direc- 
tions through  the  porous  partition,  until  both  are  thoroughly  mingled  with 
each  other. 

333.  If  a  liquid  is  placed  in  contact  with  a  surface  of 
the  body,  divested  of  its  epidermis,  or  outer  skin,  or  in 
contact  with  a  mucous  membrane,  the  liquid  will  be  ab- 
sorbed into  the  vessels  of  the  body  through  the  force  of 
endosmose. 

PRACTICAL   QUESTIONS   AND   PROBLEMS   IN  HYDROSTATICS. 

1.  Why  are  stones,  gravel,  and  sand  so  easily  moved  by  waves  and  currents  ? 
Because  the  moving  water  has  only  to  overcome  about  half  the  weight  of 

the  stone. 

2.  Why  can  a  stone  which,  on  land,  requires  the  strength  of  two  men  to  lift  it,  be 
lifted  and  carried  in  water  by  one  man  ? 

Because  the  water  holds  up  the  stone  with  a  force  equal  to  the  weight  of 
the  volume  of  water  it  displaces. 

3.  Why  does  cream  rise  upon  milk  ? 

Because  it  is  composed  of  particles  of  oily,  or  fatty  matter,  which  are  lighter 
than  the  watery  particles  of  the  milk. 

4.  How  are  fishes  able  to  ascend  and  descend  quickly  in  water  ? 

They  are  capable  of  changing  their  bulk  by  the  voluntary  distension,  or 
contraction  of  a  membraneous  bag,  or  air  bladder,  included  in  their  organiza- 
tion ;  when  this  bladder  is  distended,  the  fish  increases  in  size,  and  being  of 
less  specific  gravity,  i.  e.,  lighter,  it  rises  with  facility ;  when  the  bladder  is 
contracted,  the  size  of  the  fish  diminishes,  and  its  tendency  to  sink  is  increased. 

5.  Why  doea  the  body  of  a  drowned  person  generally  rise  and  float  upon  the  surface 
several  days  after  death  ? 

Because,  from  the  accumulation  of  gas  within  the  body  (caused  by  incipient 
putrefaction),  the  body  becomes  specifically  lighter  than  water,  and  rises  and 
floats  upon  the  surface. 

6    How  are  life-boats  prevented  from  sinking  ? 

They  contain  in  their  sides  air-tight  cells,  or  boxes,  filled  with  air,  which  by 
their  buoyancy  prevent  the  boat  from  sinking,  even  when  it  is  filled  with  water. 

7.  Why  does  blotting-paper  absorb  ink? 

The  ink  is  drawn  up  between  the  minute  fibers  of  the  paper  by  capillary 
attraction. 

8.  Why  will  not  writing,  or  sized  paper,  absorb  ink  ? 

Because  the  sizing,  being  a  species  of  glue  into  which  writing  papers  are 


148  WELLS'S  NATURAL  PHILOSOPHY. 

dipped,  fills  up  the  little  interstices,  or  spaces,  between  the  fibers,  and  in  this 
way  prevents  all  capillary  attraction. 

9.  Why  is  vegetation  on  the  margin  of  a  stream  of  water  more  luxuriant  than  in  an 
open  field  ? 

Because  the  porous  earth  on  the  bank  draws  up  water  to  the  roots  of  the 
plants  by  capillary  attraction. 

10.  Why  do  persons  who  water  plants  in  pots  frequently  pour  the  water  into  the  sau- 
cer in  which  the  pot  rests,  and  not  over  the  plants  ? 

Because  the  water  in*  the  saucer  is  drawn  up  by  capillary  attraction  through 
the  little  interstices  of  the  mold  with  which  the  pot  is  filled,  and  is  thus  pre- 
sented to  the  roots  of  the  plant. 

11.  Why  does  dry  wood,  immersed  in  water,  swell  ? 

Because  the  water  enters  the  pores  of  wood  by  capillary  attraction,  and 
forces  the  particles  further  apart  from  each  other. 

12.  Why  will  water,  ink,  or  oil,  coming  in  contact  with  the  edge  of  a  book,  soak  fur- 
ther in  than  if  spilled  upon  the  sides  ? 

Because  the  space  between  the  leaves  acts  in  the  same  manner  as  a  small 
capillary  tube  would — attracts  the  fluid,  and  causes  it  to  penetrate  far  inward. 
The  fluid  penetrates  with  more  difficulty  upon  the  side  of  the  leaf,  because 
the  pores  in  the  paper  are  irregular,  and  not  continuous  from  leaf  to  leaf. 

13.  In  a  hydrostatic  press,  the  area  of  the  base  of  the  piston  in  the  force-pump  is  one 
square  inch,  and  the  area  of  the  base  of  the  piston  in  the  large  cylinder  is  fourteen  square 
inches ;  what  will  be  the  force  exerted,  supposing  a  power  of  eight  hundred  pounds  ap- 
plied to  the  piston  of  the  force-pump  ? 

14.  A  flood-gate  is  five  feet  in  breadth,  and  sixteen  feet  in  depth:  what  will  be  the 
pressure  of  water  upon  it  in  pounds  ? 

15.  What  pressure  will  a  vessel,  having  a  superficial  area  of  three  feet,  sustain  when 
lowered  into  the  sea  to  the  depth  of  five  hundred  feet  ? 

16.  What  pressure  is  exerted  upon  the  body  of  a  diver  at  the  depth  of  sixty  feet,  sup- 
posing the  superficial  area  of  his  body  to  be  two  and  a  half  square  yards  ? 

17.  What  will  be  the  pressure  upon  a  dam,  the  area  of  the  side  of  which  is  one  hun- 
dred and  fifty  superficial  feet,  and  the  height  of  the  side  fifteen  feet,  the  water  rising  even 
with  the  top  ? 


CHAPTER    IX. 

HYDRAULICS. 

334.  HYDRAULICS  is  that  department  of 

What  is  the  .  r 

science  of  Hy-    physical  science  which  treats  of  the  laws  and 

draulics?  r    J  . 

phenomena  of  liquids  in  motion. v 

Hydraulics  considers  the  flow  of  liquids  in  pipes,  through  orifices  in  the 
sides  of  reservoirs,  in  rivers,  canals,  etc.,  and  the  construction  and  operation 
of  all  machines  and  engines  which  are  concerned  in  the  motion  of  liquids. 
*  From  t'Jw/a  (hudor),  water,  and  avX6<  (aulos),  a  pipe. 


HYDRAULICS.  149 

upon  what  does  335.  When  .an  opening  is  made  in  a  reser- 
flowTng^'ifquM  vo^r  containing  a  liquid,  it  will  jet  out  with  a 
depend?  velocity  proportioned  to  the  depth  of  the  aper- 

ture below  the  surface. 

FIG  140  Supposing  the  surface  of  -water  in  a  vessel,  D,  Fig. 

140,  to  be  kept  at  a  constant  height  by  the  water 
flowing  into  it,  and  that  the  water  flows  out  through 
openings  in  the  side  of  precisely  the  same  size ;  then 
a  quart  measure  would  be  filled  from  the  jet  issuing  from 
B  as  soon  as  a  pint  measure  from  the  upper  opening,  A. 
As  the  flow  of  liquids  is  in  consequence  of  the  at- 
traction of  gravity,  and  as  the  pressure  of  a  liquid  is 
equal  in  all  directions,  we  have  the  following  princi- 
ple established: — 

-  ^e  v^^ty  w^ich  the  particles  of  a 
ac(luire  when  issuing  from  an  orifice, 
whether  sideways,  upward,  or  downward,  is 

i  •    i  11  • 

equal  to  that  which  they  would  have  acquired 
in  falling  perpendicularly  through  a  space  equal  to  the 
depth  of  the  aperture  below  the  surface  of  the  liquid. 

Thus,  if  an  aperture  be  made  in  the  bottom,  or  side,  of  a  vessel  containing 
water,  16  feet  below  the  surface,  the  velocity  with  which  the  water  will  jet 
out  will  be  32  feet  per  second,  for  this  is  the  velocity  which  a  body  acquires 
in  falling  through  a  space  of  16  feet. 

As  the  velocity  acquired  by  a  falling  body  is  as  the  square  root  of  the  space 
.through  which  it  falls,  the  velocity  with  which  water  will  issue  from  an  aper- 
ture may  be  calculated  by  the  following  rule :— - 

HOW  may  the        337.  The  velocity  with  which  water  spouts 

Hquicdtyflowing  out  ^om  any  aperture  in  a  vessel  is  as  the 

53™  be  caicul  square  root  of  the  depth  of  the  aperture  below 

lated?  the  surface  of  the  water. 

.  The  water  must,  therefore,  flow  with  ten  times  greater  velocity  from  an 
opening  100  inches  below  the  level  of  the  liquid,  than  from  a  depth  of  only 
one  inch  below  the  same  level. 

What  is  the  ^^'  ^^e  theoretical  ^aw  f°r  determining  the  quantity  of 
theoretical  law  water  discharged  from  an  orifice  is  as  follows: — 

ing  the  quan-        The  quantity  of  water  discharged  from  an  ori- 

tit/    of   -water       «.-,-,  ,  i      ?    .      i  i_  i,- 

discharged  fice  m  each  second  may  be  calculated  by  multi- 
tureVn  plying  the  velocity  by  the  area  of  the  aperture. 

The  above  rules  for  calculating  the  velocity  and  quantity  of  water  flowing 
from  orifices,  are  not  found  strictly  to  hold  good  itf  practice.  The  friction 
of  water  against  the  sides  of  vessels,  pipes,  and  apertures,  and  the  formation 


150  WELLS'S  NATURAL  PHILOSOPHY. 

of  what  is  called  the  "contracted  vein,"  tend  very  much  to  diminish  the  mo- 
tion and  discharge  of  water.  -p, 
What  is  the  When  water  flows  through  a  circular  aperture 
"  contracted  in  a  vessel,  the  diameter  of  the  issuing  stream 
rent'of  water?  ^  contracted»  and  attains  its  smallest  dimensions 
at  a  distance  from  the  orifice  equal  to  the  diam- 
eter of  the  orifice  itself.  The  section  of  the  jet  at  this  point,  Fig. 
141,  s  s',  will  be  about  two  thirds  of  the  magnitude  of  the  onfice. 
This  point  of  greatest  contraction  is  called  the  vena  contracta,  or  contracted  vein. 
Wh  t  is  th  ^*"s  Pnenomenon  arises  from  the  circumstance  that  a  liquid 
cause  of  this  contained  in  a  vessel  rushes  from  all  sides  toward  an  orifice, 
phenomenon  ?  go  as  to  form  a  system  of  converging  currents.  These  issuing 
out  in  oblique  directions,  cause  the  shape  of  the  stream  to  change  from  the 
cylindrical  form,  and  contract  it  in  the  manner  described. 
How  ma  the  ^  ^ne  attachment  of  suitable  tubes  to  the  aperture,  the 
effect  of  the  effect  of  the  contracted  vein  may  be  avoided,  and  the  quan- 
veSlfatoid-  *%  of  flowing  water  be  very  greatly  increased.  A  short  pipe 
ed?  will  discharge  one  half  more  water  in  the  same  time,  than 
a  simple  orifice  of  the  same  dimensions.  The  tube,  however,  must  be 
FiG  142  entirely  without  the  vessel, 
as  at  B,  Fig.  142,  for  if  COY  • 
tinued  inside,  as  at  A,  the 
quantity  of  liquid  discharged 
will  be  diminished  instead 
of  augmented. 

The  rapidity  of  the  discharge  of  the  water  will  also  depend  much  on  the 
figure  of  the  tube,  and  that  of  the  bottom  of  the  vessel,  since  more  water 
will  flow  through  a  conical,  or  bell-shaped  tube,  as  at  C,  Fig.  142,  than 
through  a  cylindrical  tube.  A  still  further  advantage  may  be  gained  by  hav- 
ing the  bottom  of  the  vessel  rounded,  as  at  D,  and  the  tube  bell-shaped. 

An  inch  tube  of  200  feet  in  length,  placed  horizontally,  will  discharge  only 
one  fourth  as  much  water  as  a  tube  of  the  same  dimensions  an  inch  in 
length ;  hence,  in  all  cases  where  it  is  proposed  to  convey  water  to  a  distance 
in  pipes,  there  will  be  a  great  disappointment  in  respect  to  the  quantity  actu- 
ally delivered,  unless  the  engineer  takes  into  account  the  friction,  and  the 
turnings  of  the  pipes,  and  makes  large  allowances  for  these  circumstances. 
If  the  quantity  to  be  actually  delivered  ought  to  fill  a  two  inch  pipe,  one  of 
three  inches  will  not  be  too  great  an  allowance,  if  the  water  is  to  be  conveyed 
to  any  considerable  distance. 

In  practice,  it  will  be  found  that  a  pipe  of  two  inches  in  diameter,  one  hun- 
dred feet  long,  will  discharge-  about  five  times  as  much  water  as  one  of  one 
inch  in  diameter  of  the  same  length,  and  under  the  same  pressure.  This  dif- 
ference is  accounted  for,  by  supposing  that  both  tubes  retard  the  motion  of 
the  fluid,  by  friction,  at  equal  distances  from  their  inner  surfaces,  and  conse- 
quently, the  effect  of  this  cause  is  much  greater  in  proportion,  in  the  small 
tube,  than  hi  the  large  one. 


1,% 


HYDRAULICS. 


151 


What  will  be 
tlie  difference 
in  the  tlow  of 
a  liquid  when 
the  vessel  is 
ki>pt  full  and 
when  it  is  al- 
lowed to  emp- 
ty itself/ 


What    is    the 
principle    and 
construction 
of  the   water- 
clock  ? 


FIG.  143. 

A 


As  the  velocity  with  which  a  stream  issues  depends  upon  the  height  of  the 
column  of  fluid,  it  follows  that  when  a  liquid  flows  from  a  reservoir  which  is 
not  replenished,  but  the  level  of  which  constantly  descends,  its  velocity  will 
be  uniformly  retarded.  The  following-  principle  has  been  established : — 

339.  If  a  vessel  be  filled  with  a  liquid  and 
allowed  to  discharge  itself,  the  quantity  issu- 
ing from  an  orifice  in  a  given  time,  will  be 
just  one  half  what  would  be  discharged  from 
the  same  orifice  in  the  same  time,  if  the  vessel 
was  kept  constantly  full. 

340.  Before   the   invention   of  clocks  and 
watches,  the  flow  of  water  through  small  ori- 
fices was  applied  by  the  ancients  for  the  meas- 
urement of  time,  and  an  arrangement  for  this 

purpose  was  called  a  Ckpsydfa,  or  water-clock.  One  form  of 
this  instrument  consisted  of  a  cylindrical  vessel  filled  with 
water,  and  furnished  with  an  orifice  which  would  discharge  the 
whole  in  twelve  hours.  If  the  whole  depth  through  which  the 
water  in  the  vessel  would  sink  in  this  time  be  divided  into 
144  parts,  it  will  sink  through  23  in  the  first  hour,  21  in  the 
second,  19  in  the  third,  and  so  on,  according  to  a  series  of  odd 
numbers :  this  diminishing  rate  depending  on  the  constantly 
decreasing  height  and  pressure  of  the  column  above  the  point 
of  discharge.  The  spaces  indicated  upon  a  scale  attached  to 
the  side  of  the  vessel,  and  compared  with  the  position  of  the 
descending  column,  marks  the  time.  Fig.  143  represents  the 
form  of  the  water  clock. 

341.  The  force  of  currents,  whe- 
ther in  pipes,  canals,  or  rivers,  is 

more  or  less  resisted,  and  their  velocity  re- 
tarded, by  the  friction  which  takes  place  be- 
tween those  surfaces  of  the  liquid  and  the  solid  which  are 
in  contact. 

This  explains  a  fact  which  may  be  observed  in  all  rivers: 
that  the  velocity  of  a  stream  is  always  greater  at  the  center 
than  near  the  bank,  and  the  velocity  at  the  surface  is  greater 
than  the  velocity  at  the  bottom. 

342.  If  a  given  quantity  of  liquid  must  pass 
through  pipes  or  channels  of  unequal  section 
in  the  same  time,  its  velocity  will  increase  as 
the  transverse  section  diminishes,  and  dimin- 
ish as  the  area  of  the  section  increases. 


How  is  the  ve- 
locity of  water 
in  pipes  and 
rivers  retard- 
ed ? 


At  what  part 
of  a  stream  is 
the  velocity 
greatest  ? 

In  a  channel  of 
unequal  sec- 
tion, how  will 
the  velocity  of 
a  current  be 
affected? 


152  WELLS'S  NATURAL  PHILOSOPHY. 

This  fact  is  familiar  to  every  one  who  observes  the  course  of  brooks  or 
rivers:  wherever  the  bed  contracts,  the  current  becomes  rapid,  and  on  the 
contrary  if  it  widens,  the  stream  becomes  more  sluggish. 

343.  A  very  slight  declivity  is  sufficient  to  give  motion  to 

running  water.  Three  inches  to  a  mile  in  a  smooth,  straight 
tient  to  give  channel,  gives  a  velocity  of  about  three  miles  per  hour. 

Tlie  river  Ganges,  at  a  distance  of  1,800  miles  from  its 

mouth,  is  only  800  feet  above  the  level  of  the  sea.  The 
average  rate  of  inclination  of  the  surface  of  the  Mississippi  is  1.80  for  the  first 
hundred  miles  from  the  Gulf  of  Mexico,  2  inches  for  the  second  hundred,  2.30 
for  the  third,  and  only  2.57  for  the  fourth. 

What   is    the          Thc  velocity  of  rivers  is  extremely  variable  ;  the  slower  class 
average  veioci-      moving  from  two  to  three  miles  per  hour,  or  three  or  four  feet 

per  second,  and  the  more  rapid  as  much  as  six  feet  per  second. 
The  mean  velocity  of  the  Mississippi,  near  its  mouth,  is  2.26  miles  per  hour, 
or  2.95  feet  per  second.* 

The  quantity  of  water  which  passes  over  the  beds  of  rivers  in  a  given  time 
is  very  various.  In  the  smaller  class  of  streams  it  amounts  to  from  300  to 
350  cubic  feet  per  second.  In  the  smaller  class  of  navigable  rivers,  it  amounts 
to  from  1,000  to  1,200  cubic  feet;  and  in  the  larger  class  to  14,000  cubic  feet 
and  upward.  It  is  estimated  that  the  Mississippi  discharges  12  billions  of 
cubic  feet  of  water  per  minute,  f 

*  In  the  construction  of  water-channels  for  drainage,  the  regulation  of  inclination  neces- 
sary to  produce  free  flowage  of  the  water,  is  a  matter  of  great  importance.  This  inclination 
varies  greatly  with  the  size  of  the  stream  of  water  to  be  conducted  off.  Large  and  deep 
rivers  run  sufficiently  swift  with  a  fall  of  a  few  inches  per  mile ;  smaller  rivers  and  brooks 
require  a  fall  of  two  feet  per  mile,  or  1  foot  in  2,500.  Small  brooks  hardly  keep  an  open 
course  under  4  feet  per  mile,  or  1  in  1,200;  while  ditches  and  covered  drains  require  at 
least  8  feet  per  mile,  or  1  in  600.  Furrows  of  ridges,  and  drains  partially  filled  with  loose 
materials,  require  a  much  greater  inclination. 

t  A  question  of  some  interest  relative  to  the  course  and  flow  of  rivers,  may,  perhaps, 
be  appropriately  considered  in  this  connection.  The  question  is  as  follows:  Do  the 
Mississippi,  and  other  rivers  whose  courses  are  northerly  and  southerly,  flow  up  hill  or 
down  hill  ?  The  Mississippi  runs  from  north  to  south.  If  its  source  were  at  the  pole  and 
its  mouth  at  the  equator,  the  elevation  of  the  mouth  would  be  thirteen  miles  higher  than 
its  source,  as  this  is  the  difference  between  the  equatorial  and  the  polar  radii  of  the 
earth.  On  this  principle,  the  mouth  of  the  Mississippi  is  two  and  a  half  miles  more  ele- 
vated than  its  source.  Does  it  run  up  hill,  and  if  so,  how  has  its  course  and  motion 
originated?  The  problem,  although  apparently  one  of  difficulty,  admits  of  an  easy 
solution. 

The  centrifugal  force,  caused  by  the  rotation  of  the  earth,  has  changed  the  form  of  our 
planet  from  that  of  a  perfect  sphere  to  that  of  an  ellipsoid,  or  a  sphere  flattened  at  the 
poles,  in  which  the  length  of  the  largest  radius,  exceeds  the  shorter  by  thirteen  miles,  the 
present  form  being  the  figure  of  equilibrium  under  the  present  conditions.  The  cohesion 
of  the  solid  particles  of  the  earth  has  resisted,  and  does  resist,  to  a  limited  extent, 
the  influence  of  the  centrifugal  force  which  has  changed  the  original  figure ;  but  the  par- 
ticles of  liquid  on  the  earth's  surface,  being  perfectly  free  to  move,  yield  to  the  influence, 
and  are  at  rest  only  so  long  as  the  condition  of  equilibrium  is  undisturbed,  and  always 
move  in  such  a  way  as  to  restore  it  when  it  is  disturbed.  Water,  consequently,  always 
flows  from  places  which  are  above  the  figure  of  equilibrium,  to  those  which  are  below  it. 
Now  the  mouth  of  the  Mississippi  is  two  and  a  half  miles  more  distant  from  the  center  of 


HYDRAULICS.  153 

HOW  are  waves  344.  When  one  portion  of  a  liquid  is  dis- 
Sces  form*  turbed,  the  disturbance  (in  consequence  of  the 
ed?  freedom  with  which  the  particles  of  a  liquid 

move  upon  each  other)  is  communicated  to  all  the  other 
portions,  and  a  wave  is  formed.  This  wave  propagates 
itself  into  the  unmoved  spaces  adjoining,  continually  en- 
larging as  it  goes,  and  forming  a  series  of  undulations. 

345.  Ordinary  sea  waves  are  caused  by  the 

What    is    the  J  f  e 

wales  ?°f  6ea    wmd  Passing  unequally  upon  the  surface  or 
the  water,  depressing  one  part  more  than  an- 
other :   every  depression  causes  a  corresponding  elevation. 

Where  the  water  is  of  sufficient  depth,  waves  have  only  a 
Does  the  sub-      verticai  motion,  t.  e.,  up  and  down.     Any  floating  body,  as  a 

siJincG   01    cne  , 

wave  actually  buoy,  floating  on  a  wave,  is  merely  elevated  and  depressed 
alternately;  it  does  not  otherwise  change  its  place.  The 
apparent  advance  of  waves  in  deep  water  is  an  ocular  decep- 
tion :  the  same  as  when  a  corkscrew  is  turned  round,  the  thread,  or  spiral, 
appears  to  move  forward. 

346.  A  wave  is  a  form,  not  a  thing ;  the  form  advances,  but 
Why  do  wares  .  ,  __,    '     ,  ' . 

always    break     not  the  substance  of  the  wave.     When,  however,  a  rock  rises 

shore?  ' the  to  the  surface>  or  the  sllore  by  its  shallowness  prevents  or  re- 
tards the  oscillations  of  the  water,  the  waves  forming  in  deep 
water  are  not  balanced  by  the  shorter  undulations  in  shoal  water,  and  they 
consequently  move  forward  and  form  breakers.  Thus  it  is  that  waves  always 
break  against  the  shore,  no  matter  in  what  direction  the  wind  blows. 

When  the  shore  runs  out  very  shallow  for  a  great  extent,  the  breakers  are 
distinguished  by  the  name  of  surf. 

On  the  Atlantic,  during  a  storm,  the  waves  have  been  observed  to  rise  to 
&  height  of  about  forty-three  feet  above  the  hollow  occupied  by  a  ship ;  the 
total  distance  between  the  crests  of  two  large  waves  being  559  feet,  which 
distance  was  passed  by  the  wave  hi  about  seventeen  seconds  of  time. 

the  earth  (i.  «.,  the  center  of  figure)  than  the  source  is.  But  if  it  had  not  been  for  the 
restraining  influence  of  the  cohesive  force  prevailing  among  the  solid  particles,  it  would 
have  been,  through  the  action  of  the  centrifugal  force,  three  miles  higher,  instead  of  two 
and  a  half.  It  is  therefore  below  the  surface  of  equilibrium,  and  the  water  flows  south 
to  fill  up  the  proper  level. 

The  question  as  to  whether  the  river  flows  up,  or  down,  depends  on  the  meaning  we 
attach  to  the  words  used.  If  by  DOWN  we  mean  toward  the  earth's  center  of  figure,  or 
toward  that  part  of  the  earth's  surface  where  the  attraction  of  gravity  is  the  greatest,  as 
at  the  poles,  then  the  Mississippi  runs  up  hill.  If,  on  the  contrary,  DOWN  means  below 
the  surface  of  equilibrium,  and  UP  means  above  the  surface  of  equilibrium,  then  the  Mis. 
sissipi  flows  downward.  If  the  earth  were  a  perfect  sphere,  and  without  rotation,  the 
river  would  flow  northward.  A  more  complete  explanation  of  this  subject  will  be  found 
in  a  paper  read  before  the  American  Academy  by  Prof.  Levering  in  1856,  and  in  the 
•'  Annual  of  Scientific  Discovery"  for  1857,  pp.  179—182. 

7* 


154  WELLS'S  NATURAL  PHILOSOPHY. 

HOW  dees  the        347.  The  resistance  which  a  liquid  opposes 
iiquidatnoca  solid    to  a  solid  body  moving  through  it,  varies  with 


The  resistance  which  a  plane  surface  meets  with  while  it 
moves  in  a  liquid,  in  a  direction  perpendicular  to  its  plane,  is  in  general,  pro- 
portioned to  the  square  of  its  velocity. 

What  advan-  If  tlie  surface  of  a  solid  moved  against  a  liquid  be  presented 
tagehasanob-  obliquely  with  respect  to  the  direction  of  its  motion,  instead 
inqUemo^infgCe  of  perpendicularly,  the  resistance  will  be  modified  and  dimin- 
against  a  liq-  ished  ;  the  quantity  of  liquid  displaced  will  be  less,  and  the 
surface,  acting  as  a  wedge,  or  inclined  plane,  will  possess  a 
mechanical  advantage,  since  in  displacing  the  liquid  it  pushes  it  aside,  instead 
of  driving  it  forward. 

The  determination  of  the  particular  form  which  should  be  given  to  a  mass 
of  matter  in  order  that  it  may  move  through  a  liquid  with  the  least  resistance, 
is  a  problem  of  great  complexity  and  celebrity  in  the  history  of  mathematics, 
inasmuch  as  it  is  connected  with  nearly  all  improvements  in  navigation  and 
naval  architecture.  The  principles  involved  in  this  problem  require  that  the 
length  of  a  vessel  should  coincide  with  the  direction  of  the  motion  imparted 
to  it  ;  and  they  also  determine  the  shape  of  the  prow  and  of  the  surfaces  be- 
neath the  water.  Boats  which  navigate  still  waters,  and  are  not  intended  to 
carry  a  great  amount  of  freight,  are  so  constructed  that  the  part  of  the  bot- 
tom immersed  moves  against  the  liquid  at  a  very  oblique  angle. 

Vessels  built  for  speed  should  have  the  greatest  possible  length,  with  merely 
the  breadth  necessary  to  stow  the  requisite  cargo. 

The  form  and  structure  of  the  bodies  of  fishes  in  general,  are  such  as  to  en- 
able them  to  move  through  the  water  with  the  least  resistance. 
Wh  n  ar    th  348<  In  tne  Paddles  of  steamboats,  that  one  is  only  corn- 

paddles  of  a  pletely  effectual  in  propelling  the  vessel  which  is  vertical  in 
steamboat  most  ^  ws^er^  because  upon  that  one  alone  does  the  resistance 
of  the  water  act  at  right  angles,  or  to  the  best  advantage. 
In  the  propulsion  of  steamboats,  it  is  found  that  paddle-wheels  of  a  given 
diameter  act  with  the  greatest  effect  when  their  immersion  does  not  exceed 
the  width,  or  depth,  of  the  lowest  paddle-board  ;  their  effect  also  increases 
with  the  diameter  of  the  wheel. 

The  amount  of  power  lost  by  the  use  of  the  paddle-wheel 
Is  the  paddle-  ,  ,  ,. 

wheel  an  ad-      as  a  means  of  propelling  vessels  is  very  great,  since,  in  addi- 

™ntageouB          tion  to  the  fact  that  only  the  paddle  which  is  vertical  in  the 

method  of  ap-  * 

plying    power     water  is  fully  effective,  the  series  of  paddles  in  descending 

111118  into  tne  water,  are  obliged  to  exert  a  downward  pressure, 
which  is  not  available  for  propulsion,  and  in  ascending,  to  lift 
a  considerable  weight  of  water  that  opposes  the  ascent,  and  adheres  to  the 
paddles.  The  rolling  of  the  vessel,  also,  renders  it  impossible  to  maintain  the 
paddles  at  the  requisite  degree  of  immersion  necessary  to  give  them  their 
greatest  efficiency  ;  one  wheel  on  one  side  being  occasionally  immersed  too 


HYDRAULICS. 


155 


deeply,  while  the  other  wheel,  on  the  other  side  may  be  lifted  entirely  out  of 

water. 

349.  To  remedy  in  some  degree  these  causes  of  inefficiency 
and  waste,  the  submerged  propelling-wheel,  known  as  tho 
screw -propeller,  has  been  introduced  within  the  last  few  years. 
The  screw-propeller  consists  of  a  wheel  resembling  in  its  form 
the  threads  of  a  screw,  and  rotating  on  an  axle.  It  is  placed 

in  the  stern  of  the  vessel,  below  the  water-line,  immediately  in  front  of  the 

rudder.     Fig.  144  represents  one  form  of  the  sere w-propeller,*  and  its  location. 

in  reference  to  the  other  parts  of  the  vessel. 


Describe      the 
construction 
and    action   of 
tho  screw-pro- 
peller. 


FIG.  144. 


The  manner  in  which  the  screw-propeller  acts  in  impelling  the  vessel  for- 
ward, may  be  understood  by  supposing  the  wheel  to  be  an  ordinary  screw, 
and  the  water  surrounding  it  a  solid  substance.  By  turning  the  screw  in  one 
direction  or  the  other,  it  would  move  through  the  water,  carrying  the  vessel 
with  it,  and  the  space  through  which  it  would  move  in  each  revolution  would 
be  equal  to  the  distance  between  two  contiguous  threads  of  the  screw.  In 
fact,  the  water  would  act  as  a  fixed  nut,  in  which  the  screw  would  turn. 
But  the  water,  although  not  fixed  in  its  position  as  a  solid  nut,  yet  offers  a 
considerable  resistance  to  the  motion  of  the  screw-wheel ;  and  as  the  wheel 
turns,  driving  the  water  backward,  the  reaction  of  the  water  gives  a  propul- 
sion to  the  vessel  in  a  contrary  direction,  or  forward. 

The  great  advantage  of  the  screw-propeller  is,  that  its  ac- 
tion on  the  water  will  be  the  same,  no  matter  to  what  degree 
it  may  be  immersed  in  it,  or  how  the  position  of  the  vessel 
on  the  surface  of  the  water  may  be  changed. 

350.  The  application  of  the  force  of  water  in  motion  for  im- 
pelling machinery,  is  most  extensive  and  familiar.  The  sim- 
plest method  of  applying  this  force  as  a  mechanical  agent,  is 
by  means  of  wheels,  which  are  caused  to  revolve  by  the 


What  is  the 
great  advan- 
tage of  the 
screw-propeller 
over  the  pad- 
dle-wheel ? 


What   is  the 
simplest  meth- 
od    of     using 
water  as  a  mo- 
tive power  ? 


156 


WELLS'S   NATURAL   PHILOSOPHY. 


Into  how  many 


weight,  cr  pressure,  of  the  water  applied  to  their  circumferences.  These 
wheels  are  mounted  upon  shafts,  or  axles,  which  are  in  turn  connected  with 
the  machinery  to  which  motion  is  to  be  imparted. 

351.  The  water-wheels  at  present  most  gen- 
erally used  may  be  divided  into  four  classes — • 
divided?  the  UNDERSHOT,  the  OVERSHOT,  the  BREAST 

WHEEL,  and  the  TOURBINE  WHEEL. 

*     352.     The      Undershot  FIG.  145. 

Describe      the  „          ,      , 

constr  action  of     "Wheel  consists  of  a  wheel, 

on    the    circumference    of 

which  are  fixed  a  number 
of  flat  boards  called  "float-boards"  at  equal 
distances  from  each  other.  It  is  placed  in 
such  a  position  that  its  lower  floats  are  im- 
mersed in  a  running  stream,  and  is  set  in 
motion  by  the  impact  of  the  water  on  the 
boards  as  they  successively  dip  into  it.  A 
wheel  of  this  kind  will  revolve  in  any 
stream  which  furnishes  a  current  of  suffi- 
cient power.  Fig.  145  represents  the  construction  of  the  undershot  wheel. 

This  form  of  wheel  is  usually  placed  in  a  "race-way,"  or  narrow  passage,  in 
such  a  manner  as  to  receive  the  full  force  of  a  current  issuing  from  the  bottom 
of  a  dam,  and  striking  against  the  float-boards.  And  it  is  important  to  re- 
member, that  the  moving  power  is  the  same,  whether  water  falls  downward 
from  the  top  of  a  dam  to  a  lower  level,  or  whether  it  issues  from  an  opening 
made  directly  at  the  lower  level.  This  will  be  obvious,  if  it  is  considered 
that  the  force  with  which  water  issues  from  an  opening  made  at  any  point  in 
the  dam  will  be  equal  to  that  which  it  would  acquire  in  falling  from  the  sur- 
face or  level  of  the  water  in  the  dam  down  to  the  same  point. 

The  undershot  wheel  is  a  most  disadvantageous  method  of 

applying  the  power  of  water,  not  more  than  25  per  cent,  of 

the  moving  power   of  the  water  being  rendered  available 

by  it. 

353.    In      the      Overshot 

"Wheel,  the  water  is  received 

into  cavities  or  cells,  called 
"buckets,"  formed  in  the  circumference  of  the 
wheel,  and  so  shaped  as  to  retain  as  much  of 
the  water  as  possible,  until  they  arrive  at  the 
lowest  part  of  the  wheel,  where  they  empty 
themselves.  The  buckets  then  ascend  empty 
on  the  other  side  of  the  wheel  to  be  filled  as- 
before.  The  wheel  is  moved  by  the  weight  of 


What  propor- 
tion of  power  is 
lost  by  the  un- 
dershot wheel  ? 

Describe  the 
construction  of 
the  Overshot 
Wheel. 


FIG.  146. 


the  water  contained  in  the  buckets  on  the  descending  side. 
sents  au  overshot  wheel 


Fig.  146  repre- 


HYDRAULICS. 


157 


The  overshot  wheel  is  one  of  the  most  effective  varieties  of 
water-wheels,  and  receives  its  name  from  the  circumstance 
that  the  water  shoots  over  it.  It  requires  a  fall  in  the  stream, 
rather  higher  than  its  own  diameter.  Wheels  of  this  kind, 
when  well  constructed,  utilize  nearly  three  fourths  of  the  mov- 
ing force  of  the  water. 

354.  The  Breast  Wheel  may  be  considered  as  a  variety 
intermediate  between  the  overshot  and  the  undershot  wheels. 
In  this,  the  water,  instead  of  falling  on  the  wheel  from  above, 
or  passing  entirely  beneath  it,  is  delivered  just  below  the  level 


"What  propor- 
tion of  the 
moving  power 
is  utilized  by 
the  overshot 
wheel  ? 


Describe  the 
construction  of 
the  Breast- 
Wheel. 


PIG.  147. 


of  the  axis.  The  race-way,  or  passage  for 
the  water  to  descend  upon  the  side  of  the 
wheel,  is  built  in  a  circular  form,  to  fit  the 
circumference  of  the  wheel,  and  the  water 
thus  inclosed  acts  partially  by  its  weight, 
and  partially  by  its  impulse,  or  momentum. 
Fig.  147  represents  a  breast-wheel,  with  its 
circular  race- way. 

The  breast-wheel,  when  well  constructed, 
will  utilize  about  65  per  cent,  of  the  mov- 
ing power  of  the  water.  It  is  more  efficient 
than  the  undershot  wheel,  but  less  than  the 
overshot.  It  is  therefore  only  used  where  the  fall  happens  to  be  particularly 
adapted  for  it. 

355.  The  fourth  class  of  water-wheels,  the  "  Tour-  F™-  14=8. 

bine,"  or  "  Turbine,"  is  a  wheel  of  modern  invention, 
and  is  the  most  powerful  and  economical  of  all  water- 
engines. 

The  principles  of  the  construction  and  action  of  the 
Tourbine  wheel  may  be  best  understood  by  a  previous 
examination  of  the  construction  of  another  water- 
engine  known  as  "  Barker's  Mill."  (See  Fig.  148.) 

This  consists  of  an  upright  tube  or 
Describe     the 

construction  of  cylinder,  furnished  with  a  -  smaller 
Barker's  Mill.  crosg.tube  at  the  bottom,  and  en- 
larged into  a  funnel  at  the  top.  The  whole  cylinder 
is  so  supported  upon  pivots  at  the  top  and  bottom, 
that  it  revolves  freely  about  a  vertical  axis.  It  is 
evident  if  there  are  no  openings  in  the  ends  of  the 
cross-tubes,  and  the  whole  is  filled  with  water,  that 
the  entire  arrangement  will  be  simply  that  of  a  close 
vessel  filled  with  water,  without  any  tendency  to  motion.  If,  however,  the 
ends  of  the  arms,  or  cross-tube,  have  openings  on  the  sides,  opposite  to  one 
another,  as  is  represented  in  the  figure,  the  sides  of  the  tube  on  which  the 
openings  are,  will  be  relieved  from  the  pressure  of  the  column  of  water  in  the 
upright  tube  by  the  water  flowing  out,  while  the  pressure  on  the  sides  oppo- 


158 


WELLS'S  NATURAL  PHILOSOPHY. 


Describe     the 
construction 
and   action    of 
the     Tourbine 
Wheel. 


FIG.  149. 


site  to  them,  which  have  no  openings,  will  remain  the  same.  The  machine, 
therefore,  will  revolve  in  the  direction  of  the  greater  pressure,  that  is,  in  a 
direction  contrary  to  that  of  the  jets  of  water.  A  supply  of  water  poured  into 
the  funnel-head,  keeps  the  cylinder  full,  and  the  pressure  of  the  column  of 
water  constant. 

The  action  of  this  machine  may  also  be  explained  according  to  another 
view :  the  pressure  of  the  column  of  water  in  the  upright  tube,  will  cause  the 
water  to  be  projected  in  jets  from  the  openings  at  the  ends  of  the  arms  in 
opposite  directions ;  when  the  recoil,  or  reaction  of  these  jets  upon  the  ex- 
tremities of  the  cross-tubes,  gives  a  rotary  motion  to  the  whole  machine  upon 
its  vertical  axis. 

The  Tourbine  wheel  derives  its  motion,  like  the  Barker's 
mill,  from  the  action  of  the  pressure  of  a  column  of  water. 
It  consists  of  a  fixed,  horizontal  cylinder,  A  B,  Fig.  149,  in 
the  center  of  which  the  water  enters  from  an  upright  tube  or 
cylinder,  corresponding  in  position 
to  the  upright  cylinder  of  a  Bark- 
er's mill.  The  water  descend- 
ing in  the  tube  diverges  from  the 
center  in  every  direction,  through 
curved  water-channels,  or  com- 
partments, A  and  B,  formed  in  the 
horizontal  cylinder,  and  escapes  at 
the  circumference.  Around  the 
fixed  horizontal  cylinder,  a  hori- 
zontal wheel,  D,  in  the  form  of  a 
ring  or  circle,  is  fitted,  with  its  rim 
formed  into  compartments  exactly 
similar  to  the  compartments  of  the 
fixed  cylinder,  with  the  exception 
that  their  sides  curve  in  an  oppo- 
site direction.  The  water  issuing 
from  the  guide-curves  A  B,  strikes  against  the  curved  compartments  of  the 
wheel  C  B,  and  causes  it  to  revolve.  The  wheel,  by  attachments  beneath  the 
fixed  cylinder  A  B,  is  connected  with  a  shaft,  E,  which  passes  up  through  the 
fixed  and  upright  cylinder,  and  by  which  motion  is  imparted  to  machinery. 

The  Tourbine  wheel  may  be  used  to  advantage  with  a  fall 
efficiency   *?     of>  water  of  any  height,  and  will  utilize  more  of  the  force  of 
the    Tourbine     the  moving  power  than  any  other  wheel — amounting,  in  some 
instances,  as  at  the  cotton  factories  at  Lowell,  Mass.,  to  up- 


wheel  ? 


ward  of  95  per  cent,  of  the  whole  force  of  the  water. 

356.  It  may  appear  strange  to  those  unacquainted  with  the 
action  of  hydraulic  engines,  that  so  much  of  the  power  exist- 
ing in  the  agent  we  use  for  producing  motion,  as  running 
water,  should  be  lost,  amounting  in  the  undershot  wheel  to 
75  por  cent,  of  the  whole  power.  This  is  due  partially  to  tho 


Is  it  possible 
to  construct  a 
water-wheel 
which  will  ren- 
der the  whole 
power  availa- 
ble? 


HYDRAULICS.  159 

friction  of  the  water  against  the  surfaces  upon  which  it  flows,  and  to  the  fric- 
tion of  the  wheel  which  receives  the  force  of  the  current.  Force  is  also  lost 
by  changing  the  direction  of  the  water  in  order  to  convey  it  to  the  machinery ; 
in  the  sudden  change  of  velocity  which  the  water  undergoes  when  it  first 
strikes  the  wheels  ;  and  more  than  all,  from  the  fact,  that  a  considerable  amount 
of  force  is  left  unemployed  in  the  water  which  escapes  with  a  greater  or  less 
velocity  from  every  variety  of  wheel.  It  may  be  considered  as  practically 
impossible  to  construct  any  form  of  water-engine  which  will  utilize  the  whole 
force  of  a  current  of  water. 

357.  "Water,  although  one  of  the  most  abundant  substances  in  nature,  and 
a  universal  necessity  of  life,  is  not  always  found  in  the  location  in  which  it  is 
desirable  to  use  it.  Mechanical  arrangements,  therefore,  adapted  to  raise 
water  from  a  lower  to  a  higher  level,  have  been  among  the  earliest  inventions 
of  every  country. 

what  were  &58.  The  application  of  the  lever,  in  the 
£gS|££f£  form  of  the  old-fashioned  well-sweep  (still 
raising  water?  us^  fa  many  parts  Of  this  C0untry,  and 

throughout  Eastern  Asia),  of  the  pulley  and  rope,  and 
the  wheel  and  axle  in  the  form  of  the  windlass,  were  un- 
doubtedly the  earliest  mechanical  contrivances  for  raising 
water. 

The  screw  of  Archimedes,    invented  by  the  philosopher 
Describe     the  J 

Archimedes  whose  name  it  bears,  is  a  contrivance  for  raising  water,  of 
screw.  great  antiquity. 

This  machine,  represented  in  Fig. 
FIG.  150.  150^  consists  Of  a  tube  wound  in  a 

spiral  form  about  a  solid  cylinder,  A 
B,  which  is  made  to  revolve  by  turn- 
ing the  handle  H.  This  cylinder  is 
placed  at  a  certain  inclination,  with 
its  lower  extremity  resting  in  the 
water.  As  the  cylinder  is  made  to 
revolve,  the  end  of  the  tube  dips  into 
the  water,  and  a  certain  portion  en- 
ters the  orifice  a.  By  continuing 
the  revolution  of  the  cylinder,  the 
water  flows  down  a  series  of  inclined 
planes,  or  to  the  under  side  of  the 
tube,  and  if  the  inclination  of  the 

tube  be  not  too  great,  the  water  will  finally  flow  out  at  the  upper  orifice  into 

a  proper  receptacle. 

The  following  diagram,  Fig.  151,   representing  the  curved  tube  in  two 

opposite  positions,  will  illustrate  the  action  of  the  Archimedes  screw.    Suppose 

a  marble  dropped  into  the  tube  at  o>  fig.  1, :  if  it  was  kept  stationary  in  the 


160 


WELLS'S  NATURAL  PHILOSOPHY. 


FiO.  151.  tube  until  it  -was  turned  half  round,  as  in  the 

position,  fig.  2,  the  marble  would  be  at  of]  now, 
if  at  liberty  to  move,  it  would  roll  down  to  &'; 
but  this  effect,  which  we  have  supposed  accom- 
plished all  at  once,  is  really,  gradually  performed, 
and  a  rolls  down  toward  b'  by  the  gradual  turn- 
ip ing  of  the  tube,  and  reaches  b'  as  soon  as  the 
screw  comes  into  the  position  marked  in  fig.  2 ; 
another  half  turn  of  the  screw  would  bring  it 
into  its  first  position,  and  the  marble  would 

gradually  roll  forward  to  c. 

When  was  the          359>  The  common  suction-pump  is  a  later  discovery  than  the 

common  pump  screw  of  Archimedes,  and  is  supposed  to  have  been  invented 
by  Ctesibius,  an  Athenian  engineer  who  lived  at  Alexandria, 

in  Egypt,  about  the  middle  of  the  second  century  before  the  Christian  era.* 


Describe  the 
construction  of 
the  chain-pump. 


FlG.    152. 


360.  The  chain-pump 
consists  of  a  tube,  or  cyl- 
inder, the  lower  part  of 
which  is  immersed  in  a  well  or  reser- 
voir, and  the  upper  part  enters  the  bot- 
tom of  a  cistern  into  which  the  water  is 
to  be  raised.  An  endless  chain  is  car- 
ried round  a  wheel  at  the  top,  and  is 
furnished  at  equal  distances  with  flat 
discs,  or  plates,  which  fit  tightly  in  the 
tube.  As  the  wheel  revolves,  they  suc- 
cessively enter  the  tube,  and  carry  the 
water  up  before  them,  which  is  dis- 
charged into  the  cistern  at  the  top  of  the 
tube.  The  machine  may  be  set  in  mo- 
tion by  a  crank  attached  to  the  upper 
wheel. 

Fig.  152  represents  the  construction 
and  arrangement  of  the  chain-pump. 

in  what  sitna-         The  chain-pump   will 
tions    is    this      act  with  its  greatest  ef- 

fect>   when    the   Cylinder 
in  which  the  plates  and 

chain  move,  can  be  placed  in  an  inclined 
position,  instead  of  vertically.  It  is  used 
generally  on  board  of  ships  and  in  sit- 
uations where  the  height  through  which 
the  water  is  to  be  elevated  is  not  very  great,  as  iu  cases  where  the  founda- 
tions of  docks,  etc.,  are  to  be  drained. 

*  The  suction-pump,  and  other  machines  for  raising  water  which  depend  upon  the 
pressure  of  the  atmosphere,  are  described  under  the  head  of  Pneumatic*. 


HYDRAULICS. 


161 


For  what  other 
purposes  than 
raising  water 
is  the  chain- 
pump  used  ? 


What  is  an  Hy- 
draulic Ram? 


Describe    the 
construction  of 
the  Hydraulic 

liilltt. 


FIG.  153. 


This  machine  is /not,  however,  used  exclusively  for  raising 
"water.  Its  application,  in  principle,  may  be  seen  in  any  grist- 
mill, where  it  conveys  the  flour  discharged  from  the  stones, 
to  an  upper  part  of  the  building,  where  it  is  bolted.  Dredg- 
ing machines  for  elevating  mud  from  the  bottom  of  rivers,  aro 
also  constructed  on  the  same  principle, 

361.  The  HYDRAULIC  KAM  is  a  machine 
constructed  to  raise  water  by  taking  advantage 
of  the  impulse,  or  momentum,  of  a  current  of  water  sud- 
denly stopped  in  its  course,  and  made  to  act  in  another 
direction. 

The  simplest  construction  of  the  hydraulic  ram  is  repre- 
sented in  Fig.  153,  and  its  operation  is  as  follows : — At  the 
end  of  a  pipe,  B,  connected  with  a  spring,  or  reservoir,  A, 
somewhat  elevated,  from  which  a  supply  of  water  is  derived, 
is  a  valve,  E,  of  such  weight  as  just  to  fall  when  the  water  is  quiet,  or  still, 

within  the  pipe ;  this  pipe  is  com 
nected  with  an  air-chamber,  D, 
from  which  the  main  pipe,  F,  leads  ; 
this  air-chamber  is  provided  with 
a  valve  opening  upward,  as  shown 
in  the  cut.  Suppose  now,  the 
water  being  still  within  the  tubef 
the  valve  E  to  open  by  its  own 
weight;  immediately  the  stream 
begins  to  run,  and  the  water  flow- 
ing through  B  soon  acquires  a 
momentum,  or  force,  sufficient  to 

raise  the  valve  E  up  against  its  seat.  The  water,  being  thus  suddenly  ar- 
rested in  its  passage,  would  by  its  momentum  burst  the  pipe,  were  it  not  for 
the  other  valve  in  the  air-chamber,  D,  which  is  pressed  upward,  and  allows 
the  water  to  escape  into  the  air-chamber,  D.  The  air  contained  in  the 
chamber  D  is  condensed  by  the  sudden  influx  of  the  water,  but  immediately 
reacting  by  means  of  its  elasticity,  forces  a  portion  of  the  water  up  into  the 
tube  F. 

As  soon  as  the  water  in  the  pipe  B  is  brought  to  a  state  of  rest,  the  valve 
of  the  air-chamber  closes,  and  the  valve  E  falls  down  or  opens ;  again  the 
stream  commences  running,  and  soon  acquires  sufficient  force  to  shut  the 
valve  E ;  a  new  portion  is  then,  by  the  momentum  of  the  stream,  urged  into 
the  air-chamber  and  up  the  pipe  F ;  and  by  a  continuance  of  this  action, 
water  will  be  continually  elevated  in  the  pipe  F. 

Fig.  154  represents  a  more  improved  construction  of  the  ram.  in  which  by 
the  use  of  two  air-chambers,  C  and  F,  the  force  of  the  machine  is  greatly  in- 
creased. A  represents  the  main  pipe,  B  the  valve  from  whence  the  water 
escapes,  G  the  pipe  in  which  it  is  elevated. 


162  WELLS'S   NATURAL  PHILOSOPHY. 

FIG.  154. 


As  this  machine  produces  a  kind  of  intermitting  motion  from  the  alternate 
flux  and  reflux  of  the  stream,  accompanied  with  a  noise  arising  from  the  shock, 
its  action  has  been  compared  to  the  butting  of  a  ram ;  and  hence  the  name  of 
the  machine. 

It  will  be  seen  from  these  details,  that  a  very  insignificant  pressing  column 
of  water,  running  in  the  supply  pipe,  is  capable  of  forcing  a  stream  of  water 
to  a  very  great  height,  so  that  a  sufficient  fall  of  water  may  be  obtained  in  any 
running  brook,  by  damming  up  its  upper  end  to  produce  a  reservoir,  and  then 
carrying  the  pipe  down  the  channel  of  the  stream  until  a  sufficient  fall  is 
obtained.  A  considerable  length  of  descending  pipe  is  desirable  to  insure  the 
action  of  the  stream,  otherwise  the  water,  instead  of  entering  the  air-vessel, 
may  be  thrown  back,  when  the  valve  is  closed,  into  the  reservoir. 


What   is   the 


Into  what  two 
classes  may  all 
aeriform  sub- 
stances be  di- 
vided ? 


CHAPTER   X. 

PNEUMATICS. 


362.  PNEUMATICS  is  that  department   of 
physical  science  which  treats  of  the  motion 
and  pressure  of  air,*  and  other  aeriform,  or 

gaseous  substances. 

363.  Aeriform,  or  gaseous  bodies,  may  be 
divided  into  two  classes,  viz.,  the  permanent 
gases,  or  those  which  under  all  ordinary  cir- 
cumstances of  temperature  and  pressure  are 

always  in  the  gaseous  state,  as  common  air  ;  and  the  va- 
pors, which  may  readily  be  condensed  by  pressure,  or  the 
diminution  of  temperature,  into  liquids,  as  steam,  or  the 
vapor  of  water. 

364.  Atmospheric  air  is  taken  as  the  type,  or  representative,  of  all  perma- 
nent gases,  and  steam  as  the  type  of  all  vapors,  because  these  substances  pos- 
sess the  general  properties  of  gases  and  vapors  in  the  utmost  perfection. 

what  is  the  365.  The  atmosphere  is  a  thin,  transparent 
atmosphere?  flui^  or  aeriform  substance,  surrounding  the 
earth  to  a  considerable  height  above  its  surface,  and  which 
by  its  peculiar  constitution  supports  and  nourishes  all 
forms  of  animal  or  vegetable  life. 

*  Atmospheric  air  is  composed  of  oxygen  and  nitrogen  mixed  together  in  the  proportion 
of  seventy-nine  parts  of  nitrogen  and  twenty-one  of  oxygen,  or  about  four-fifths  nitrogen 
to  one-fifth  oxygen.  These  two  gases  existing  in  the  atmosphere  are  not  chemically  com- 
bined with  each  other,  but  merely  mixed. 

Beside  these  two  ingredients  there  is  always  in  the  air,  at  all  places,  carbonic  acid  ga« 
and  watery  vapor,  in  variable  proportions,  and  sometimes  also  the  odoriferous  matter  of 
flowers,  and  other  volatile  substances. 

The  air  in  all  regions  of  the  earth,  and  at  all  elevations,  never  varies  in  compositior,  so 
fai  as  regards  the  proportions  of  oxygen  and  nitrogen  which  it  contains,  no  matter  whether 
it  te  collected  on  the  top  of  high  mountains,  over  marshes,  or  over  deserts. 

It  is  a  wonderful  principle,  or  law  of  nature,  that  when  two  gases  of  different  weights, 
or  specific  gravities,  are  mixed  together,  they  can  not  remain  separate,  as  fluids  of  differ- 
ent densities  do,  but  diffuse  themselves  uniformly  throughout  the  whole  space  which  both 
occupy.  It  is,  therefore,  by  thia  law  that  a  vapor,  arising  by  its  own  elasticity  from  a 
volatile  substance,  is  caused  to  extend  its  influence  and  mingle  with  the  surrounding  at- 
mosphere, until  its  effects  become  so  enfeebled  by  dilution  as  to  be  imperceptible  to  the 
senses.  Thus  we  are  enabled  to  enjoy  and  perceive  at  a  distance  the  odor  of  a  flower- 
garden,  or  a  perfume  which  has  been  exposed  in  an  apartment. 


164  WELLS'S  NATURAL   PHILOSOPHY. 

The  atmosphere  is  not,  as  is  generally  regarded,  invisible. 
^nen  seen  through  a  great  extent,  as  when  we  look  upward 
in  the  sky  on  a  clear  day,  the  vault  appears  of  an  azure,  or 
deep  blue  color.  Distant  mountains  also  appear  blue.  In  both  these  instances 
the  color  is  due  to  the  great  mass  of  air  through  which  we  direct  our  vision. 

The  reason  that  we  do  not  observe  this  color  in  a  small  quan- 
ra  small  quanti-  tity  of  air  is,  that  the  portion  of  colored  light  reflected  to  the 
Llbif  color  ?CX"  e^e  ky  a  h'rafted  quantity  is  insufficient  to  produce  ihe  requis- 
ite sensation  upon  the  eye,  and  in  this  way  excite  in  the  mind 
-a  perception  of  the  color.  Almost  all  slightly  transparent  bodies  are  exam- 
ples of  this  fact. 

If  a  glass  tube  of  small  bore  be  filled  with  sherry  wine,  or  wine  of  a  simi- 
lar color,  and  looked  at  through  the  tube,  it  will  be  found  to  have  all  the 
appearance  of  water,  and  be  colorless.  If  viewed  from  above,  downward,  in 
the  direction  of  its  length,  it  will  be  found  to  possess  its  original  color.  In 
the  first  instance,  there  can  be  no  doubt  that  the  wine  has  the  same  color 
as  the  liquid  of  which  it  originally  formed  a  part  ;  but  in  the  case  of  small 
quantities,  the  color  is  transmitted  to  the  eye  so  faintly,  as  to  be  inadequate  to 
produce  perception.  For  the  same  reason,  the  great  mass  of  the  ocean 
appears  green,  while  a  small  quantity  of  the  same  water  contained  in  a  glass 
is  perfectly  colorless. 

Does  air  pos-  366.  Air,  in  common  with  other  material 

sentiai1  'quail",  substances,  possesses  all  the  essential  quali- 

ties of  matter?  faes  of  matter,  as  impenetrability,  inertia,  and 
weight. 

367.  The  impenetrability  of  air  may  be  shown  by  taking  a 
"What          aro 
proofs  of  the     hollow  vessel,  as  a  glass  tumbler,  and  immersing  it  in  water 


with-its  mouth  downward  ;  it  will  be  found  that  the  water 
will  not  fill  the  tumbler.  If  a  cork  is  placed  upon  the  water 
under  the  mouth  of  the  tumbler,  it  will  be  seen  that  as  the  tumbler  is  pressed 
down,  the  air  in  it  will  depress  the  surface  of  the  water  on  which  the  cork 
floats.  The  diving-bell  is  constructed  on  the  same  principle. 

368.  The  inertia  of  the  air  is  shown  by  the  resistance  which 
proofs  of  the  it  opposes  to  the  motion  of  a  body  passing  through  it.  Thus, 
inertia  of  air  ?  jf  we  open  an  umbrella,  and  endeavor  to  carry  it  rapidly  with 
the  concave  side  forward,  a  considerable  force  will  be  required  to  overcome 
the  resistance  it  encounters.  A  bird  could  not  fly  in  a  space  devoid  of  air, 
even  if  it  could  exist  without  respiration,  since  it  is  the  inertia,  or  resistance 
of  the  particles  of  the  atmosphere  to  the  beating  of  the  wings,  which  enables 
it  to  rise.  The  wings  of  birds  are  larger,  in  proportion  to  their  bodies,  than 
the  fins  of  fishes,  because  the  fluid  on  which  they  act  is  less  dense,  and  lias 
proportionally  less  inertia,  than  the  water  upon  which  the  fins  of  fishes  act 

TO  what  ex-        3(59.  Air  is  highly  compressible  and  perfectly 

tent      is      air       plactip 
compressible?       CiaStlC. 

By  these  two  qualities  air  and  all  other  gaseous  substances 


PNEUMATICS.  165 

are  particularly  distinguished  from  liquids,  which  resist  compression,  and  pos- 
sess but  a  small  degree  of  elasticity,  illustrations  of  the  compressibility  of  air 
are  most  familiar.  A  quantity  of  air  contained  in  a  bladder,  or  India-rubber 
bag,  may  be  easily  forced  by  the  pressure  of  the  hand,  to  occupy  less  space. 
There  is,  indeed,  no  theoretical  limit  to  the  compression  of  air,  for  with  every 
additional  degree  of  force,  an  additional  degree  of  compression  may  be  obtained. 
The  elasticity,  or  expansibility  of  air,  also  manifests  itself 
•topu*  conl  in  an  unlimited  degree.  Air  cannot  be  said  to  have  any 

stant  size  or  or;0-inal  size  or  volume,  for  it  always  strives  to  occupy  a 
volume  ? 

larger  space. 

What  are  illus-  When  a  part  of  the  air  inclosed  in  any  vessel  is  withdrawn, 
expansibility16  that  which  remains,  expanding  by  its  elastic  property,  always 
of  air  ?  fiUs  the  dimensions  of  the  vessel  as  completely  as  before.  If 

nine  tenths  were  withdrawn,  the  remaining  one  tenth  would  occupy  the  same 
space  that  the  whole  did  formerly. 

This  tendency  of  air  to  occupy  a  larger  space,  or  in  other  words,  to  increase 
its  volume,  causes  it  when  confined  in  a  vessel,  to  continually  press  against 
the  inner  surface.  If  no  corresponding  pressure  acts  from  the  outer  surface, 
the  air  will  burst  it,  unless  the  vessel  is  of  considerable  strength.  This  fact  may 
be  shown  by  the  experiment  of  placing  a  bladder  partially  filled  with  air  be- 
neath the  receiver  of  an  air-pump,  and  by  exhausting  the  air  in  the  receiver 
the  pressure  of  the  external  air  upon  the  outer  surface  of  the  bladder  is  re- 
moved. The  elasticity  of  the  air  contained  in  the  bladder  being  then  unre- 
sisted  by  any  external  pressure,  will  dilate  the  bladder  to  its  fullest  extent, 
and  oftentimes  burst  it. 

369.  Air,  as  well  as  all  other  gases  and  va- 

Has air  weight?  .    ,   J 

pors,  possesses  weight. 

The  weight  of  air  may  be  shown  by  first  weighing  a  suitable  vessel  filled 
with  air ;  then  exhausting  the  air  from  it  by  means  of  an  air-pump,  and  weigh- 
ing again.  The  difference  between  the  two  weights  will  be  the  weight  of  the 
air  contained  in  the  vessel. 

The  weight  of  100  cubic  inches  of  air  is  about  31  grains. 

370.  The  elasticity,  or  expansion  of  air  is  due 

To  what  is  the  .  /»     ,  i          i         /> 

elasticity     of    to  the  peculiar  action  of  the  molecular  forces 

air  due  ? 

among  its  particles,  which  manifest  themselves 
in  a  very  different  manner  from  what  they  do  in  solid  and 
liquid  bodies. 

In  solid  bodies,  these  forces  hold  the  molecules,  or  particles  together  so 
closely,  that  they  can  not  change  their  respective  positions ;  they  also  hold 
together  the  particles  of  liquid  bodies,  but  to  such  a  limited  extent  only,  as  to 
enable  the  particles  to  move  upon  each  other  with  perfect  freedom.  But  in 
gases,  or  aeriform  substances,  the  molecular  forces  act  repulsively,  and  give  to  the 
particles  a  tendency  to  move  away  from  each  other;  and  this  to  so  great  an  ex- 
tent, that  nothing  but  external  impedimenta  can  hinder  their  further  expansion. 


166  WELLS'S  NATURAL  PHILOSOPHY. 

The  question,  therefore,  naturally  occurs  in  this  connection, 
What       limits  \ 

the  atmosphere  viz.:  If  air  expands  unlimitedly,  when  unrestricted,  why  does 
to  the  earth?  not  our  atmosphere  leave  the  earth  and  diffuse  itself  through- 
out space  indefinitely  ?  This  it  would  do  were  it  not  for  the  action  of  gravi- 
tation. The  particles  of  air,  it  must  be  remembered,  possess  weight,  and  by 
gravity  are  attracted  toward  the  center  of  the  earth.  This  tendency  of  gravity  to 
condense  the  air  upon  the  earth's  surface,  is  opposed  by  the  mutual  repulsion 
existing  between  the  particles  of  air.  These  two  forces  counterbalance  each 
other :  the  atmosphere  will  therefore  expand,  that  is,  its  particles  will  separate 
from  one  another,  until  the  repulsive  force  is  diminished  to  such  an  extent  as  to 
render  it  equal  to  the  weight  of  the  particles,  or  what  is  the  same  thing,  to 
the  force  of  the  attraction  of  gravitation,  when  no  further  expansion  can  take 
place.  We  may  therefore  conceive  the  particles  of  air  at  the  upper  surface  of 
the  atmosphere  resting  in  equilibrium,  under  the  influence  of  two  opposite 
forces,  viz.,  their  own  weight,  tending  to  carry  them  downward,  and  tlxe 
mutual  repulsion  of  the  particles,  which  constitutes  the  elasticity  of  air,  tend- 
ing to  drive  them  upward. 

what  law  reg-  371.  The  density  of  the  air,  or  the  quantity 

rityoVthedCai~  contained  in  a  given  bulk,  decreases  with  the 

mosphere?  altitude,  or  height  above  the  surface  of  the 
earth. 

This  is  owing  to  the  diminished  pressure  of  the  air,  and  FIG.  155. 

the  decreasing  force  of  gravity.  Those  portions  directly 
incumbent  upon  the  earth  are  most  dense,  because  they  bear 
the  weight  of  the  superincumbent  portions;  thus,  the  hay 
at  the  lower  part  of  the  stack  bears  the  weight  of  that 
above,  and  is  therefore  more  compact  and  dense.  (See  Fig. 
155.)  This  idea  may  be  conveyed  by  the  gradual  shading 
of  the  figure,  which  indicates  the  gradual  diminution  in  the 
density  of  the  atmosphere  in  proportion  to  its  altitude. 

when  is  air         372.  Air  is  said  to  be  rarefied 

saiil  to  be  rare-  ,  .  ••      ,  ,  •• 

fied?  when   it  is  caused  to  expand  and  occupy  a 

greater  space. 

Generally,  when  we  speak  of  rarefied  air,  we  mean  air  that  is  expanded  to 
a  greater  degree,  or  is  thinner,  than  the  air  at  the  immediate  surface  of  the 
earth. 

373.  The  great  law  governing  the  compressibility  of  air,  which  is  known 
from  its  discoverer  as  "Mariotte's  Law,"  may  be  stated  as  follows: 

what  is  Ma-  The  volume  of  space  which  air  occupies  is  in- 
riotta's  Law?  versely  as  the  pressure  upon  it. 

If  the  compressing  force  be  doubled,  the  air  which  is  compressed  will 
occupy  one  half  of  the  space :  if  the  compressing  force  be  increased  in  a  three- 
fold proportion,  it  will  occupy  one  third  the  space ;  if  fourfold,  one  fourth  the 
space,  and  so  on. 


PNEUMATICS. 


167 


The  relation  between  the  compressibility  of  air,  and  its  elasticity  and  dens- 
ity, also  obeys  a  certain  law  which  may  thus  be  expressed : — 

374.  The  density  and  elasticity  of  air  are 
directly  as  the  force  of  compression. 

This  relation  is  clearly  exhibited  by  the  following  table : — 
With  the  same  amount  of  air,  occupying  the  space  of 
i    i    j    i    i    i      i 

**    2>    3>    4?   T>J   IT)   77T7)J 

the  elasticity  and  density  will  be  1,  2,  3,  4,  5,  6,  100. 

Hence  by  compressing  air  into  a  very  small  space,  by  means 
of  a  proper  apparatus,  we  can  increase  its  elastic  force  to  such 
an  extent  as  to  apply  it  for  the  production  of  very  powerful 
effects.  The  well-known  toy,  the  pop-gun,  is  an  example  of 
the  application  of  this  power.  The  space  A  of  a  hollow  cylinder,  Fig.  156,  is 
inclosed  by  the  stopper,  p,  at  one  end,  and  by  the  end  of  the  rod,  S,  at  the 
other  end.  This  rod  being  pushed  further  into  the  cylinder,  the  air  contained 
in  the  space,  A,  is  compressed  until  its  elastic  force  becomes  so  great  as  to 
drive  out  the  stopper,  p,  at  the  other  end  of  the  cylinder  with  great  force, 

FIG.  156. 


"What  relation 
exists  between 
the  compressi- 
bility of  air 
and  "its  elastic- 
ity and  density  ? 


lustrations   of 
of cjj*sticforce 


accompanied  with  a  report     The  air-gun  is  constructed  and  operated  on  a 


similar  principle. 

Prove  and   il  3  ^5*  The  1&WS  °f  Mariotte  mav  te 

lustrate  the  illustrated  and  proved  by  the  following 
laws  ofMariotte.  experiment:  let  A  B  C  D  be  a  long, 
bent  glass  tube,  open  at  its  longer  extremity,  and  fur- 
nished with  a  stop-cock  at  the  shorter.  The  stop-cock 
being  open  so  as  to  allow  free  communication  with 
the  air,  a  quantity  of  mercury  is  poured  into  the 
open  end.  The  surfaces  of  the  mercury  will,  of  course, 
stand  at  the  same  level,  E  F,  in  both  legs  of  the 
tube,  and  will  both  sustain  the  weight  of  a  col- 
umn of  air  reaching  from  E  and  F  to  the  top  of 
the  atmosphere.  If  we  now  close  the  stop-cock,  D, 
the  effect  of  the  weight  of  the  whole  atmosphere 
above  that  point  is  cut  off,  so  that  the  surface,  F,  can 
sustain  no  pressure  arising  from  the  weight  of  the 
atmosphere.  Still,  the  level  of  the  mercury  in  both 
legs  of  the  tube  remains  the  same,  because  the  elas- 
ticity of  the  air  inclosed  in  F  D  is  precisely  equal,  and 
sufficient  to  balance  the  weight  of  the  whole  column 


FIG.  157. 


1G8  WELLS'S   NATURAL   PHILOSOPHY. 

of  atmosphere  pressing  upon  the  surface,  E.  If  this  were  not  the  case,  or  if 
there  were  no  air  in  F  D,  then  the  weight  of  the  atmosphere  pressing  upon 
the  surface  E  would  force  the  mercury,  E  B  C  F,  up  into  the  space,  F  D.  The 
elasticity  of  air  is,  therefore,  directly  proportionate  to  the  force,  or  compression, 
exerted  upon  it. 

It  is  evident  that  the  pressure  exerted  upon  the  surface,  E,  Fig.  157,  what- 
ever may  be  its  amount,  is  that  of  a  column  of  air  reaching  from  E  to  the  top 
of  the  atmosphere,  or,  as  we  express  it,  the  weight  of  one  atmosphere.  The 
amount  of  this  pressure,  accurately  determined,  is  equal  to  the  weight,  or 
pressure,  which  a  column  of  mercury  30  inches  high  would  exert  on  the 
same  surface.  If  then,  we  pour  into  the  tube,  A  E,  Fig.  157,  as  much 
mercury  as  will  raise  the  surface  in  the  leg  A  B  30  inches  above  the 
surface  of  the  mercury  in  the  leg  D  C.  we  shall  have  a  pressure  on  the 
surface  of  E  equal  to  two  atmospheres;  and  since  liquids  transmit  pressure 
equally  in  all  directions,  the  same  pressure  will  be  exerted  on  the  air  included 
in  the  leg  D  F.  This  will  reduce  it  in  volume  one  half,  or  compress  it  into 
half  the  space,  and  the  mercury  will  rise  in  the  leg  D  F  from  F  to  F'.  This 
weight  of  two  atmospheres  reduces  a  given  quantity  of  air  into  one  half  its 
volume.  In  the  same  manner,  if  mercury  be  again  poured  into  the  tube  A 
E  until  the  surface  of  the  column  in  A  E  is  60  inches  above  the  level  of  the 
mercury  in  D  F,  then  the  air  in  D  F  will  be  compressed  into  one  third  of  its 
original  volume.  In  the  same  manner  it  could  be  shown,  by  continuing  these 
experiments,  that  the  diminution  of  the  volume  of  air  will  always  be  in  the 
exact  proportion  of  the  increase  of  the  compressing  force,  and  its  volume  can 
also  be  increased  in  exact  proportion  to  the  diminution  of  the  compressing 
force.  In  fact  this  law  has  been  verified  by  actual  experiment,  until  the  air 
has  been  condensed  27  times  and  rarefied  112  times. 

Air  has  been  allowed  to  expand  into  more  than  2,000  times  its  bulk,  and 
it  would  have  expanded  still  more  if  greater  space  had  been  allowed.  Air 
has  also  been  compressed  into  less  than  a  thousandth  of  its  usual  bulk,  so  as 
to  become  denser  than  water.  In  this  state  it  still  preserved  its  gaseous  form 
and  condition. 

376.  The  fact  that  air  possesses  weight,  and  consequently 
of  air  known  exerts  pressure,  was  not  known  until  about  two  hundred  years 
cients?6  *""  a&a  TIie  ancient  philosophers  recognized  the  fact,  that  air 
was  a  substance,  or  a  material  thing,  and  they  also  noticed 
that  -when  a  solid,  or  a  liquid,  was  removed,  that  the  air  rushed  in  and  filled 
up  the  space  that  had  been  thus  deserted.  But  when  called  to  give  a 
reason  for  this  phenomenon,  they  said  "  that  nature  abhorred  an  empty 
space,"  or  a  "  vacuum,"  and  therefore  filled  it  up  with  air,  or  some  liquid,  or 
solid  body. 

what  is  a  377.  A  vacuum  is  a  space  devoid  of  matter; 

vacuum?  fa  genera^  we  mean  by  a  vacuum  a  space  de- 
void of  air. 

No  perfect  vacuum  can  be  produced  artificially ;  but  confined  spaces  can 
be  deprived  of  air  sufficiently  for  all  experimental  and  practical  purposes. 


PNEUMATICS.  169 

We  do  not  know,  moreover,  that  any  vacuum  exists  in  nature,  although  there 
is  no  conclusive  evidence  that  the  spaces  between  the  planets  are  filled  with 
any  material  substance. 

If  we  dip  a  pail  into  a  pond,  and  fill  it  with  water,  a  hole  (or  vacuum)  is 
made  in  the  pond  as  big  as  tlie  pail ;  but  the  moment  the  pail  is  drawn  out, 
the  hole  is  filled  up  by  the  water  around  it.  In  the  same  manner  air  rushes 
in,  or  rather  is  pressed  in  by  its  weight,  to  fill  up  an  empty  space. 

When  we  place  one  end  of  a  straw,  or  tube  in  the  mouth, 
water  rise  in  a  and  the  other  end  in  a  liquid,  we  can  cause  the  liquid  to  rise 
tionT  ty  SU°"  m  ^ie  s^raw>  or  tube  by  sucking  it  up,  as  it  is  called.  We, 
however,  do  no  such  thing ;  we  merely  draw  into  the  mouth 
the  portion  of  air  confined  in  the  tube,  and  the  pressure  of  the  external  air 
which  is  exerted  on  the  surface  of  the  liquid  into  which  the  tube  dips,  being 
no  longer  balanced  by  the  elasticit}r  of  the  air  in  the  tube,  forces  the  liquid  up 
into  the  mouth.  If,  however,  the  straw  were  gradually  increased  in  length, 
we  should  find  that  above  a  certain  length  we  should  not  be  able  to  raise 
water  into  the  mouth  at  all,  no  matter  how  small  the  tube  might  be  in  diam- 
eter ;  or,  in  other  words,  if  we  made  the  tube  34  feet  long,  we  should  find 
that  no  power  of  suction,  even  by  the  most  powerful  machinery  instead  of 
the  moutli,  could  raise  the  water  to  that  height.  The  water  rises  in  the  com- 
mon pump  in  the  same  way  that  it  does  in  the  straw ;  but  not  above  a  height 
of  33  or  34  feet  above  tho  level  of  the  reservoir. 

378.  The  reason  why  water  thus  rises  in  a  straw,  or  pump, 
ascentof  water  remained  a  mystery  until  explained  and  demonstrated  by  Tor- 
tionUbfirsbylUx~  ricelli'  a  pupil  of  Galileo.  It  is  clear  that  the  water  is  sus- 
plained  and  <le-  tained  in  the  tube  by  some  force,  and  Torricelli  argued  that 
monstratcd?  whatever  it  might  be,  the  weight  of  the  column  of  water  sus- 
tained must  be  the  measure  of  the  power  thus  manifested;  consequently,  if 
another  liquid  be  used,  heavier  or  lighter,  bulk  for  bulk,  than  water,  then  the 
same  force  must  sustain  a  lesser  or  greater  column  of  such  liquid.  By  using  a 
much  heavier  liquid,  the  column  sustained  would  necessarily  be  much  shorter, 
and  the  experiment  in  every  way  more  manageable. 

Torricelli  verified  his  conclusions  in  the  following  manner: — He  selected 
for  his  experiment  mercury,  the  heaviest  known  liquid.  As  this  is  13-£ 
times  heavier  than  water,  bulk  for  bulk,  it  followed  that  if  the  force  imputed 
to  a  vacuum  could  sustain  33  feet  of  water,  it  would  necessarily  sustain 
13^-  times  less,  or  about  30  inches  of  mercury.  Torricelli  therefore  made  the 
following  experiment,  which  has  since  become  memorable  in  the  history  of 
science : — 

He  procured  a  glass  tube  (Fig.  158)  more  than  30  inches  long,  open  at  one 
end,  and  closed  at  the  other.  Filling  this  tube  with  mercury,  and  applying 
his  finger  to  the  open  end,  so  as  to  prevent  its  escape,  he  inverted  it,  plung- 
ing the  end  into  mercury  contained  in  a  cistern.  On  removing  the  finger,  he 
observed  that  the  mercury  in  the  tube  fell,  but  did  not  fall  altogether  into  the 
cistern ;  it  only  subsided  until  its  surface  was  at  a  height  of  about  30  inches 
above  the  surface  of  the  mercury  in  the  cistern.  The  result  was  what  Tor- 

8 


170 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  158. 


ricelli  expected,  and  he  soon 
perceived  the  true  cause  of  the 
phenomenon.  The  weight  of 
the  atmosphere  acting  upon 
the  surface  of  the  mercury  in 
the  vessel,  supports  the  liquid 
in  the  tube,  this  last  being 
protected  from  the  pressure  of 
the  .  atmosphere  by  the  closed 
end  of  the  tube. 

379.  The  fact 
How  was    the 

conclusion  of  that  the  COl- 
TorricelH  fur-  f 

ther  verified  ? 

cury    m     the 

tube  was  sustained  by  the 
pressure  of  the  atmosphere, 
was  further  verified  by  an  ex- 
periment made  by  Pascal  in 
France.  He  argued,  that  if 
the  cause  which  sustained  the 
column  in  the  tube  was  the 
weight  of  the  atmosphere  act- 
ing on  the  external  surface  of 
the  mercury  in  the  cistern, 
then,  if  the  tube  was  trans- 
ported to  the  top  of  a  high 
mountain,  where  a  less  quan- 
tity of  atmosphere  was  above 
it,  the  pressure  would  be  less, 
and  the  length  of  the  column  less.  The  experiment  was  tried  by  carrying 
the  tube  to  the  top  of  a  mountain  in  the  interior  of  France,  and  correctly 
noting  the  height  of  the  column  during  the  ascent.  It  was  noticed  that  the 
height  of  the  column  gradually  diminished  as  the  elevation  4o  which  the 
instrument  was  carried  increased. 

The  most  simple  way  of  proving  that  the  column  of  mercury  contained  in 
the  tube,  as  in  Fig.  158,  is  only  balanced  against  the  equal  weight  of  a  column 
of  air,  is  to  take  a  tube  of  sufficient  length,  and  having  tied  over  one  end  a 
bladder,  to  fill  it  up  with  mercury,  and  invert  it  in  a  cup  of  the  same  liquid; 
the  mercury  will  now  stand  at  the  height  of  about  30  inches ;  but  if  with  a 
needle  we  make  a  hole  in  the  bladder  closing  the  top  of  the  tube,  the  mer- 
cury in  the  tube  immediately  falls  to  the  level  of  that  in  the  cup. 

These  experiments  by  Torricelli  led  to  the  invention  of  the 
Barometer.  It  was  noticed  that  a  column  of  mercury  sus- 
tained in  a  tube  by  the  pressure  of  the  atmosphere,  the  tube 
being  kept  in  a  fixed  position,  as  in  Fig.  159,  fluctuated  from 
day  to  day,  within  certain  small  limits.  This  effect  was 


How  did  the 
experiment  of 
Torricelli  lead 
to  the  inven- 
tion of  the  Ba- 
rometer ? 


PNEUMATICS. 


171 


Why  should 
the  presence  of 
condensed  va- 
por of  water  in 
the  atmos- 
phere affect  its 
pressure  ? 


naturally  attributed  to  the  variation  in  the  weight  or  pres- 
sure of  the  incumbent  atmosphere,  arising  from  various  me- 
teorological causes. 

Thus,  when  the  air  is  moist  or  filled  with  vapors,  it  is  lighter 
than  usual,  and  the  column  of  mercury  stands  low  in  the 
tube ;  but  when  the  air  is  dry  and  free  from  vapor,  it  is  heavier, 
and  supports  a  longer  column  of  mercury. 

So  long  as  the  vapor  of  water  exists  in  the 
atmosphere,  as  a  constituent  part  of  it,  it  con- 
tributes to  the  atmospheric  pressure,  and  thus 
a  portion  of  the  column  of  mercury  in  the  ba- 
rometer tube  is  sustained  by  the  weight  of  the 
vapor ;  but  when  the  vapor  is  condensed,  and 
takes  on  a  visible  form,  as  clouds,  etc.,  then  it  no  longer 
forms  a  constituent  part  of  the  atmosphere,  any  more  than  dust, 
smoke,  or  a  balloon  floating  in  it  dQes,  and  the  atmospheric 
pressure  being  diminished,  the  mercury  in  the  tube  falls.  In 
this  \vay  the  barometer,  by  showing  variations  in  the  weight 
of  the  air,  indicates  also  the  changes  in  the  weather. 

380.  The  space  above  the  mercury  in  the 
barometer  tube,   A  D,  Fig.  159,  is  called  the 
Torricellian  vacuum,  and  is  the  nearest  approach  to  a  perfect 
vacuum  that  can  be  procured  by  art ;  for  upon  pressing  the 
lower  end  deeper  in  the  mercury,  the 
whole  tube  becomes  completely  filled ;  the  fluid  again 
falling  upon  elevating  the  tube,  it  is  therefore  a  per- 
fect vacuum,  with  the  exception  of  a  small  portion 
of  mercurial  vapor. 

381.  Barometers  are  constructed  in  very  different 
forms — the  principle  remaining  the  same,  of  course,  in 
all.  The  first  barometer  constnicted  was  simply  a  tube 
closed  at  one  end,  filled  with  mercury,  and  inverted 
in  a  vessel  containing  mercury,  as  in  Fig.  159. 

.  A  very  common  form  of  barometer, 

construction  of     called  the  "Wheel-Barometer,"  con- 

Baromellr  ?8l~      sists  °f  a  &laSS  tube»  bent  at  the  bot" 
torn,   and   filled  with  mercury.    (See 

Fig.  160.)  The  column  of  mercury  in  the  long  arm 
of  the  tube  is  sustained  by  the  pressure  of  the  atmos- 
phere upon  the  surface  of  the  mercury  in  the  shorter 
arm,  the  end  of  which  is  open.  A  small  float  of  iron 
or  glass  rests  upon  the  mercury  in  the  shorter  arm  of 
the  tube,  and  is  suspended  by  a  slender  thread,  which 
is  passed  round  a  wheel  carrying  an  index,  or  pointer. 
As  the  level  of  the  mercury  is  altered  by  a  variation 
of  the  pressure  of  the  atmosphere,  the  float  resting 


What  is  the 
most  perfect 
vacuum  with 
which  we  are 
acquainted  ? 


FIG.  160. 


172 


WELLS'S   NATURAL   PHILOSOPHY. 


upon  the  open  surface,  is  raised  or  lowered  in  the  FIG.  161. 

tube,  moving  the  index  over  a  dial-plate,  upon  which 
the  various  changes  of  the  weather  are  lettered. 

Fig.  1GO  represents  the  internal  structure  of  tho 
wheel-barometer,  and  Fig.  161  its  external  appear- 
ance, or  casing,  with  a  thermometer  attached. 

Describe  the  ^  very  curious  barometer,  called 
Aneroid  Ba-  the  "Aneroid  Barometer"  has  been 
rometer.  invented  and  brought  into  use  within 

the  last  few  years.      Fig.  162  respresents  its  ap- 
pearance and  construction.     Its  action  is  dependent 
on  the  effect  produced  by  atmospheric  pressure  on  a 
FiQ.162.    "  metal  box,  from 

which  the  air 
has  been  ex- 
hausted. In  the 
interior  of  the 
box  is  a  circu- 
lar spring  of 
metal,  fastened 
at  one  extremi- 
ty to  the  sides 
of  the  box,  and 
attached  at  the 
other  extremity 
by  a  suitable  ar- 
rangement to  a 
pointer,  which 
moves  over  a 
dial-plate,  or 
scale.  The  in- 
terior of  the  box  being  deprived  of  air,  the  atmospheric  pressure  upon  the 
external  surfaces  of  the  metal  sides  is  very  great,  and  as  the  pressure  varies, 
these  surfaces  will  be  elevated  and  depressed  to  a  slight  degree.  This  motion 
is  communicated  to  the  spring  in  the  interior,  and  from  thence  to  the  pointer, 
which,  moving  upon  the  dial,  thus  indicates  the  changes  in  the  weather,  or 
the  variation  in  the  pressure  of  the  atmosphere. 

What  ar    th  Water,  or  some  other  liquid  than  mercury,  may  be  used  for 

uliaritiesof  filling  the  tube  of  a  barometer.  But  as  water  is  13^  times 
lighter  than  mercury,  the  height  of  the  column  in  the  water- 
barometer  supported  by  atmospheric  pressure,  will  be  13£  times 
greater  than  that  of  mercury,  or  about  34  feet  high ;  and  a  change  which 
would  produce  a  variation  of  a  tenth  of  an  inch  in  a  column  of  mercury,  would 
produce  a  variation  of  an  inch  and  a  third  in  the  column  of  water.  The 
water-barometer  is  rarely  used,  for  various  reasons,  one  of  which  is,  that  a 
barometer  34  feet  high  is  unwieldy  and  difficult  to  transport. 


the   water-ba 
rometer  ? 


PNEUMATICS.  173" 

382.  The  ordinary  use  of  the  barometer  on  land  as  a  weather 
value*  of  the  indicator  is  extremely  limited  and  uncertain.  It  has  been 
barometer  as  a  already  stated  that  the  weight  of  100  cubic  inches  of  air  is 
cator?er  about  30  grains.  To  obtain  this  result,  it  is  necessary  that  the 

experiment  should  be  performed  at  the  level  of  the  sea,  and  it 
is  also  requisite  that  the  temperature  of  the  air  should  be  about  60°  Fahren- 
heit's thermometer,  and  that  the  height  of  the  column  of  mercury  in  the  ba- 
rometer tube  should  be  30  inches.  As  these  conditions  vary,  the  weight,  or 
pressure  of  the  atmosphere,  and  consequently  the  height  of  the  mercury  in 
the  barometer  tube  must  also  vary.  Especially  will  the  height  of  the  mer- 
curial column  vary  with  every  change  in  the  position  of  the  instrument  as 
regards  its  elevation  above  the  level  of  the  sea.  A  barometer  at  the  base  of 
a  lofty  tower  will  be  higher  at  the  same  moment  than  one  at  the  top  of  the 
tower,  and  consequently  two  such  barometers  would  indicate  different  com- 
ing changes  in  the  weather,  though  absolutely  situated  in  the  same  place.  No 
correct  judgment,  therefore,  can  be  formed  relative  to  the  density  of  the  at- 
mosphere as  affecting  the  state  of  the  weather,  without  reference  to  the  situ- 
ation of  the  instrument  at  the  tune  of  making  the  observation.  Consequently, 
no  attention  ought  to  be  paid  to  the  words  "fair,  rain,  changeable,"  etc.,  fre- 
quently engraved  on  the  plate  of  a  barometer,  as  they  will  be  found  no  cer- 
tain indication  of  the  correspondence  between  the  heights  marked,  and  the 
state  of  the  weather. 

The  barometer,  however,  may  be  generally  relied  on  for 
To  what  extent 
may  the    ba-      furnishing  an  indication  of  the  state  of  the  weather  to  this  ex- 

HeTo^forfb're-  tent  5~  that  a  fal1  of  the  mercury  in  the  tube  shows  the  ap- 
teiling  changes  proach  of  foul  weather,  or  a  storm;  while  a  rise  indicates 
intbeweather?  the  approach  of  fair  weather. 

At  sea,  the  indications  of  the  barometer  respecting  the  weather,  are  gener- 
ally considered,  from  various  circumstances,  more  reliable  than  on  land:  the 
great  hurricanes  which  frequent  the  tropics,  are  almost  always  indicated,  some 
time  before  the  storm  occurs,  by  a  rapid  fall  of  the  mercury. 
How  ma  the  383<  ^  a  barometer  be  taken  to  a  point  elevated  above  the 
barometer  be  surface  of  the  earth,  the  mercury  in  the  tube  will  fall  ;  because 


Sling'  ethe"  as  we  ascend  above  the  level  of  the  sea>  the  Pressure  of  the 
height  of  atmosphere  becomes  less  and  less.  In  this  way  the  barometer 
may  be  used  to  determine  the  heights  of  mountains,  and  tables 
have  been  prepared  showing  the  degrees  of  elevation  corresponding  to  the 
amount  of  depression  in  the  column  of  mercury. 

what  is  the  384.  The  absolute  height  to  which  the  at- 
mosphere  extends  above  the  surface  of  the 
earth  is  not  certainly  known.  There  are  good 

reasons,  however,  for  believing  that  its  height  does  not 

exceed  fifty  miles. 

Thia  envelope  of  air  is  about  as  thick,  in  proportion  to  the  whole  globe,  as 


174 


WELLS'S   NATURAL   PHILOSOPHY. 


the  liquid  layer  adhering  to  an  orange  after  it  has  been  dipped  in  water,  is 
to  the  entire  mass  of  the  orange.  Of  the  whole  bulk  of  the  atmosphere,  the 
zone,  or  layer  which  surrounds  the  earth  to  the  height  of  nearly  2  3-4  miles 
from  its  surface,  is  supposed  to  contain  one  half.  The  remaining  half  being 
relieved  of  all  superincumbent  pressure,  expands  into  another  zone,  or  belt, 
of  unknown  thickness.  Fig.  163  will  convey  an  idea  of  the  proportion  which 
the  highest  mountains  bear  to  tlie  curvature  of  the  earth,  and  the  thickness 
of  the  atmosphere.  The  concentric  lines  divide  the  atmosphere  into  six  layers, 
containing  equal  quantities  of  air,  showing  the  great  compression  of  the  lower 
layers  by  the  weight  of  those  above  them. 

FIG.  163. 


HIMALAYAS. 


Water  is  about  840  times  the  weight  of  air,  taken  bulk  for 
comparative         bulk,  and  the  weight  of  the  whole  atmosphere  enveloping  our 

globe  has  been  estimated  to  be  equal  to  the  weight  of  a  globe 

of  lead  sixty  miles  in  diameter. 
-    If  the  whole  air  were  condensed,  so  as  to  occupy  no  more  space  than  the 
same  weight  of  water,  it  would  extend  above  the  earth  to  an  elevation  of 
thirty-four  feet. 

385.  All   aeriform,  or  gaseous   substanc'es, 

How   is   the         ,.-,..,  .  7  .  ,.         ' 

pressure     of     like  liquids,  transmit  pressure  in  every  direc- 

aeriform    sub-         .  ,,  '         .  .      *    •  ,  , 

stances  exert-    tion  equally ;  therefore,  the  atmosphere  presses 
upward,  downward,  laterally,  and  obliquely, 
with  the  sarnie  force. 

386.  The  amount  of  pressure  which  the  at- 
mosphere exerts  at  the  level  of  the  ocean  is 
equal  to  a  force  of  15  pounds  for  every  square 
inch  of  surface. 

The  surface  of  a  human  body,  of  average  size,  measures 
about  2,000  square  inches.  Such  a  body,  therefore,  sustains 
a  pressure  from  the  atmosphere  amounting  to  30,000  pounds, 
or  about  15  tons. 

The  reason  we  are  not  crushed  beneath  so  enormous  a  load, 
is  because  the  atmosphere  presses  equally  in  all  directions, 
and  our  bodies  are  filled  with  liquids  capable  of  sustaining 
pressure,  or  with  air  of  the  same  density  as  the  external  air; 


What  is  the 
amount  of 
pressure  ex- 
erted by  the 
atmosphere  ? 


What  pressure 
is  sustained 
by  the  human 
body* 


Why  are  we 
not  crushed  by 
the  pressure  of 
the  atmosphere? 


PNEUMATICS. 


175 


go  that  the  external  pressure  is  met  and  counterbalanced  by  the  internal  re- 
sistance. 

If  a  man,  or  animal  were  at  once  relieved  of  all  atmospheric  pressure,  all 
the  blood  and  fluids  of  the  body  would  be  forced  by  expansion  to  the  surface, 
and  the  vessels  would  burst. 

Persons  who  ascend  to  the  summits  of  very  high  mountains, 
What  effsct  is  * 

experienced  ia     or  who  rise  to  a  great  elevation  m  a  balloon,  have  experienced 
i sing  to  great      ftlQ  most  intense  suffering  from  a  diminution  of  the  atmos- 
pheric pressure.     The  air  contained  in  the  vessels  of  the  body, 
being  relieved  in  a  degree  of  the  external  pressure,  expands,  causing  intense 
pain  in  the  eyes  and  ears,  and  the  minute  veins  of  the  body  to  swell  and 
open.     Travelers,  in  ascending  the  high  mountains  of  South  America,  have 
noticed  the  blood  to  gush  from  the  pores  of  the  body,  and  the  skin  in  many 
places  to  crack  and  burst. 

"We  become  painfully  sensible  of  the  effect  of  withdrawing 
principle  of  the  external  pressure  of  the  atmosphere  from  a  portion  of  the 
'  cupping  ?"  skin  of  the  body  in  the  operation  of  cupping.  This  is  effected 
in  the  following  manner:  a  vessel  with  an  open t mouth  is  connected  with  a 
pump,  or  apparatus  for  exhausting  the  air.  The  mouth  of  the  vessel  is  ap- 
plied in  air-tight  contact  with  the  skin ;  and  by  working  the  pump  a  part  of 
the  air  is  withdrawn  from  the  vessel,  and  consequently  the  skin  within  the 
vessel  is  relieved  from  its  pressure.  All  other  parts  of  the  body  being  still 
subjected  to  the  atmospheric  pressure,  and  the  elastic  force  of  the  fluids  con- 
tained in  the  body  having  an  equal  degree  of  tension,  that  part  of  the  skin 
which  is  thus  relieved  from  the  pressure  swells  out,  and  will  have  the  ap- 
pearance of  being  sucked  into  the  cupping-glass. 

If  the  lips  be  applied  to  the  back  of  the  hand,  and  the  breath  drawn  in  so 
as  to  produce  a  partial  vacuum  in  the  mouth,  the  skin  will  be  drawn,  or  sucked 
in — not  from  any  force  resident  in  the  lips  or  the  mouth  drawing  the  skin  in, 
but  from  the  fact  that  the  usual  external  pressure  of  air  is  removed,  and  the 
pressure  from  within  the  skin  is  allowed  to  prevail. 

Wh  do  we  of  The  sense  of  °PPression  and  lassitude  experienced  in  sum- 
ton  feel  op-  mer  previous  to  a  storm,  is  caused  by 
arsToSrm  ?bef°re  a  diminished  pressure  of  the  atmosphere. 
The  external  air,  in  such  instances,  be- 
comes greatly  rarefied  by  extreme  heat  and  by  the  con- 
densation of  vapor,  and  the  air  inside  us  (seeking  to 
become  of  the  same  rarity)  produces  an  oppressive  and 
suffocating  feeling. 

D  387.  The  direct  effects  of  atmospheric 

common"  suck-  pressure  may  be  illustrated  by  many 
er>  practical  experiments.  If  a  piece  of 

moist  leather,  called  a  sucker,  Fig.  1G4,  be  placed  in 
close  contact  with  any  heavy  body,  such  as  a  stone,  or  a 
piece  of  metal,  it  will  adhere  to  it,  and  if  a  cord  be  at- 
tached to  the  leather,  the  stone,  or  metal,  may  be  raised 


FIG.  164. 


176  WELLS'S   NATURAL   PHILOSOPHY. 

by  it.  The  effect  of  the  sucker  arises  from  the  exclusion  of  the  air  between 
the  leather  and  the  surface  of  the  stone.  The  weight  of  the  atmosphere 
presses  their  surfaces  together  with  a  force  amounting  to  15  pounds  on  every 
square  inch  of  the  surface  of  contact.  If  the  sucker  could  act  with  full 
effect,  a  disc  an  inch  square  would  support  a  weight  of  15  pounds ;  two 
square  inches,  30  pounds,  etc.  The  practical  effect,  however,  of  the  sucker 
is  much  less. 

388.  The  power  of  flies  and  other  small  insects  to  walk  on 
principle  are  ceilings,  and  surfaces  presented  downward,  or  upon  smooth 
wafk^oiTthe  Panes  of  glass,  in  opposition  to  the  gravity  of  their  bodies,  is 
ceiling,  etc.  ?  generally  refered  to  a  sucker-like  action  of  the  palms  of  their 

feet.  Recent  investigations  have,  however,  proved,  that  the  effect  is  rather 
due  to  the  mechanical  action  of  certain  minute  hairs  growing  upon  the  feet, 
which  are  tubular  and  excrete  a  sticky  liquid. 

Explain  the  389*  FoT  the  PurP°se  of  exhibiting  the  effects  produced  by 

principle  and  the  atmosphere  in  different  conditions,  and  for  various  practi- 
theSexhaustlng  ca*  PurP°ses>  instruments  have  been  contrived  by  which  air 
syringe  and  air-  may  be  removed  from  the  interior  of  a  vessel,  or  condensed 
into  a  small  space  to  any  extent,  within  certain  limits.  The 
first  of  these  requirements  may  be  obtained  by  the  use  of  the  instruments 
known  as  the  exhausting  syringe  and  the  air-pump. 

The  exhausting  syringe  consists  of  a  hollow  cylinder,  generally  FiG.  165. 
of  metal,  B  C,  Fig.  165,  very  truly  and  smoothly  bored  upon  the 
inside,  and  having  a  piston  moving  in  it  air-tight.  This  cylinder 
communicates  by  a  screw  and  pipe  at  the  bottom,  with  any  ves- 
sel, generally  called  a  receiver,  from  which  it  is  desirable  to  with- 
draw the  air.  The  piston  has  a  valve  at  E,  opening  upward, 
and  at  the  bottom  of  the  cylinder  another  valve  precisely  similar 
is  placed,  which  also  opens  upward,  shown  at  A.  Suppose 
now  the  piston  to  be  at  the  bottom  of  the  cylinder  and  the  re- 
ceiver to  be  in  proper  connection — upon  raising  the  piston  by 
the  handle,  D,  a  vacuum  is  made  in  the  cylinder ;  immediately 
the  air  in  the  receiver  expands,  passes  through  the  valve  A  at 
the  bottom  of  the  cylinder,  and  fills  its  interior ;  upon  depressing 
the  piston,  the  valve  E  opening  upward  permits  the  air  to  pass 
through,  and  the  valve  A  at  the  bottom  of  the  cylinder  closing, 
prevents  it  from  passing  back  into  the  receiver.  Upon  again 
raising  the  piston,  a  further  portion  of  air  expanding  from  the 
receiver,  enters  the  interior  of  the  syringe,  and  upon  depressing  the  piston, 
passes  out  through  its  valve.  It  is  evident  that  this  operation  may  be  con- 
tinued as  long  as  the  air  within  the  receiver  has  elasticity  sufficient  to  force 
open  the  valves. 

The  process  of  removing  air  from  a  vessel,  or  receiver,  by  means  of  the  ex- 
hausting syringe  is  slow  and  tedious,  and  more  powerful  instruments,  known 
as  air-pumps,  are  generally  employed  for  this  purpose.  The  modern  form 
of  constructing  the  air-pump  is  represented  by  Pig.  166.  The  principle  of  ita 


PNEUMATICS. 


177 


construction  is  the  same  as 
that  of  the  exhausting  sy- 
ringe, the  piston  being  work- 
ed by  a  lever  or  handle,  as  in 
the  common  pump,  the  valves 
opening  and  closing  with 
great  nicety  and  perfection. 

What  I.  the  381.  When 
construction  of  the  density 

Of  the  air  is 


FIG.  166. 


required    to 

be  increased,  the  condensing 
syringe,  the  converse  of  the 
exhausting  syringe,  is  em- 
ployed. It  consists  merely 
of  an  exhausting  syringe,  or 
air-pump,  reversed,  its  valves 
being  so  arranged  as  to  force 
air  into  a  chamber,  instead  of 
drawing  it  out.  For  this 
purpose,  the  valves  open 
inward  in  respect  to  the  interior  of  the  cylinder,  while  in  the  exhausting 
syringe  and  air-pump,  they  open  outward. 


"What  is  an  ex- 
perimental 
proof  of  the 
crushing  force 
of  the  atmos- 
phere ? 


382.  That  the  air  in  the  inside  of 
vessels  is  the  force  which  resists  and 
counterbalances  the  great  pressure 
of  the  external  atmosphere,  may  be 
proved  by  the  following  experiment : 
A  strong  glass  vessel,  Fig.  167,  is  provided,  open 
both  at  top  and  bottom,  and  having  a  diameter  of 
four  or  five  inches.  Upon  one  end  is  tied  a  bladder, 
so  as  to  be  completely  air-tight,  while  the  other  end  is 
placed  upon  the  plate  of  an  air-pump.  Upon  exhaust- 
ing the  air  from  beneath  the  bladder,  it  will  be  forced 
inward  by  the  pressure  of  the  air  outside,  and  when  the 
exhaustion  has  been  carried  to  such  an  extent  that  the 
strength  of  the  bladder  is  less  than  this  pressure,  it  will 
burst  with  a  loud  report. 

What   is    the         383'  The  air'PumP  was   invented,   in 
experiment  of     the  year  1654,  by  Otto  Guericke,  a  Ger- 

man>  and  at  a  Sreat  ?ublic  exhibition  of 
its  powers,  made  in  the  presence  of  the 
emperor  of  Germany,  the  celebrated  experiment  known 
as  the  "  Magdeburg  Hemispheres,"  was  first  shown.  The 
Magdeburg  Hemispheres,  so  called  from  the  city  where 
Guericke  resided,  consist  of  two  hollow  hemispheres  of 

8* 


FIG.  let. 


FIG.  168. 


178 


WELLS'S  NATURAL  PHILOSOPHY. 


FIG.  169. 


FIG.  170. 


brass,  Fig.  168,  which  fit  together  air-tight.  By  exhausting  the  air  in  their 
interior,  by  means  of  the  air-pump,  and  a  stop-cock  arrangement  affixed  to 
one  of  the  hemispheres,  it  will  be  found  that  they  can  not  be  pulled  apart 
without  the  exertion  of  a  very  great  force,  since  they  will  be  pressed  to- 
gether with  a  force  of  15  pounds  for  every  square  inch  of  their  surface. 
In  the  exhibition  above  referred  to,  given  of  these  hemispheres  by  Guericke, 
the  surfaces  of  a  pair  constructed  by  him  were  so  large,  that  thirty  horses] 
fifteen  upon  a  side,  were  unable  to  pull  them  apart.  By  admitting  the  air 
again  to  their  interior,  the  Magdeburg  hemispheres  fall  apart  by  their  own 
weight. 

Another  interesting  example  of  atmospheric  pressure  is, 
to  fill  a  wine-glass,  or  tumbler  with  water  to  the  brim, 
and,  having  placed  a  card  over  the  mouth,  to  invert  it 
cautiously.  If  the  card  be  kept  in  a  horizontal  position, 
the  water  will  be  supported  in  the  glass  by  the  pressure 
of  the  air  against  the  surface  of  the  card.  (See  Fig.  169.) 

Describe     the  384>  In   a  like   manner»   if  we   take   a 

principle    and     jar,  and  having  filled  it  with  water,  in- 

toe  gasometen      vert  *  in  a  reservoir  or  trough,  as  is  rep- 
resented in  Fig.  170,  it  will  continue  to.be 

completely  filled  with  water,  the  li- 
quid being  sustained  in  it  by  the  pres- 
sure of  the  atmosphere  upon  the  water 
in  the  vessel  Such  an  arrangement 
enables  the  chemist  to  collect  and  pre- 
serve the  various  gases  without  admix- 
ture  with  air ;  for  if  a  pipe  or  tube 
through  which  a  gas  is  passing  be 
depressed  beneath  the  mouth  of  the 
jar,  so  that  the  bubbles  may  rise  into 
it,  they  will  displace  the  water,  and  be 
collected  in  the  upper  part  of  the  jar, 
free  of  all  admixture. 

The  gasometers,  or  large  cylindrical 
vessels  in  which  gas  is  collected  in 
gas-works  for  general  distribution,  are 
constructed  on  this  principle.  They 
consist,  as  is  shown  in  Fig.  171,  of  a 
large  cylindrical  reservoir  suspended  with  its  mouth  downward,  and  plunged 
in  a  cistern  of  water  of  somewhat  greater  diameter.  A  pipe  which  leads 
from  the  gas-works  is  carried  through  the  water,  and  turned  upward,  so  as  to 
enter  the  mouth  of  the  gasometer.  The  gas,  flowing  through  the  pipe,  rises 
into  the  gasometer,  filling  the  upper  part  of  it,  and  pressing  down  the  water. 
Another  pipe,  descending  from  the  gasometer  through  the  water,  is  continued 
to  the  service  pipes,  which  supply  the  gas.  The  gasometer  is  balanced  by 
counter  weights  supported  by  chains,  which  pass  over  pulleys,  and  just  such 


PNEUMATICS. 


179 


a  preponderance  is  allowed  i,o  it  as  is  sufficient  to  give  the  gas  contained  in 
it  the  compression  necessary  to  drive  it  through  the  pipes  to  the  remotest  part 
of  the  district  to  be  illuminated. 

FIG.  171. 


ill  not 
liquid    flow 
from    a    tight 
cask  with  only 
one  opening  ? 


385.  A  liquid  will  not  flow  continuously  from  a  tight  cask 
after  it  has  been  tapped  or  pierced,  unless  another  opening 
is  made  as  a  vent-hole,  in  the  upper  part  of  the  cask.  The 
cask  being  air-tight,  with  the  exception  of  a  single  opening 
the  surface  of  the  liquid  in  the  vessel  will  be  excluded  from 
the  atmospheric  pressure,  and  it  can  only  flow  out  in  virtue  of  its  own 
weight.  But  if  the  weight  of  the  liquid  be  less  than  the  force  of  the  air  press- 
ing upon  the  mouth  of  the  opening,  the  liquid  can  not  flow  from  the  cask ;  the 
moment,  however,  that  the  air  is  enabled  to  act  through  the  vent-hole  in  the 
upper  part  of  the  cask,  the  pressure  below  is  counterbalanced,  and  the  liquid 
descends  and  runs  freely  through  the  opening  by  its  own  weight,. 

If  the  lid  of  a  tea-pot  or  kettle  be  air-tight,  the  liquid  will  not  flow  freely 
from  the  spout,  on  account  of  the  atmospheric  pressure.  This  is  remedied  by 
making  a  small  hole  in  the  lid,  which  allows  the  air  to  enter  from  without 

The  Pneumatic  Ink-stand,  de- 
signed to  prevent  the  ink  from 
thickening,  by  the  exposure  of  a 
small  surface  only  to  the  air,  is 
constructed  upon  the  principles 
of  atmospheric  pressure.  It  consists  of  a  close 
glass  vessel,  represented  in  Fig.  172,  from  the 
bottom  of  which  a  short  tube  proceeeds,  the 
depth  of  which  is  sufficient  for  the  immersion 
of  the  pen.  By  filling  the  ink-stand  in  an  inclined  position,  we  exclude  the 


mat  ia  the 
principle  and 
construction  of 
the  Pneumatic 
Ink-stand  ? 


FIG.  172. 


180  WELLS'S  NATURAL  PHILOSOPHY. 

air  in  great  part  from  the  interior,  and  on  replacing  it  in  an  upright  position, 
the  ink  will  be  prevented  from  rising  in  the  small  tube  and  flowing  over,  on 
account  of  the  atmospheric  pressure  upon  the  exposed  surface  of  the  ink  in 
the  small  tube,  which  is  much  greater  than  the  pressure  of  the  column  of 
liquid  in  the  interior  of  the  vessel.  As  the  ink  in  the  small  tube  is  consumed 
by  use,  its  surface  will  gradually  fall ;  a  small  bubble  of  air  will  enter  and 
rise  to  the  top  of  the  bottle,  where  it  will  exert  an  elastic  pressure,  which 
causes  the  surface  of  the  ink  in  the  short  tube  to  rise  a  little  higher,  and  this 
effect  will  be  repeated  until  all  the  ink  in  the  bottle  has  been  used. 

386.  The  peculiar  gurgling  noise  produced  when  liquid  is 
bottle    gurgle      freety  poured  from  a  bottle,  is  produced  by  the  pressure  of  the 
when  a  liquid      atmosphere  forcing  air  into  the  interior  of  the  bottle.     In  the 
ly  ou"r<>fit?ce"      first  instance,  the  neck  of  the  bottle  is  filled  with  liquid,  so  as 

to  stop  the  admission  of  air.  When  a  part  has  flowed  out, 
and  an  empty  space  is  formed  within  the  bottle,  the  atmospheric  pressure 
forces  in  a  bubble  of  air  through  the 'liquid  in  the  neck,  which  by  rushing 
suddenly  into  the  interior  of  the  bottle,  produces  the  sound.  The  bottle  will 
continue  to  gurgle  so  long  as  the  neck  continues  to  be  choked  with  liquid. 
But  as  the  contents  of  the  bottle  are  discharged,  the  liquid,  in  flowing  out, 
only  partially  fills  the  neck ;  and,  while  a  stream  passes  out  through  the  lower 
half  of  the  neck,  a  stream  of  air  passes  in  through  the  upper  part.  The  flow 
being  now  continued  and  uninterrupted,  no  sound  takes  place. 

387.  Water,  and  most  liquids  exposed  to  the  air,  absorb  a 
DiuVater  ?iSt      greater  or  less  quantity  of  it,  which  is  maintained  in  them  by 

the  pressure  of  the  atmosphere  acting  on  their  surfaces. 

:  Boiled  water  is  flat  and  insipid,  because  the  agency  of  heat  expels  the  air 
which  the  water  previously  contained.  Fishes  and  other  marine  animals 
could  not  live  in  water  deprived  of  air. 

The  presence  of  air  in  water  may  be  shown  by  placing  a 
presence  of  air  tumbler  containing  this  liquid  under  the  receiver  of  an  air- 
showrf?er  te  PumP>  and  exhausting  the  air.  The  pressure  of  the  air  being 

removed  from  the  surface  of  the  water,  minute  bubbles  will 
make  their  appearance  in  the  whole  mass  of  the  water,  and  rising  to  the  sur- 
face, escape. 

The  reason  that  certain  bottled  liquors  froth  and  sparkle 
Wny  do  some 

bottled  liquids  when  uncorked  arid  poured  into  an  open  vessel  is,  that  when 
froth  and  spar-  they  are  bottled>  the  air  confined  under  the  cork  is  condensed, 
and  exerts  upon  the  surface  a  pressure  greater  than  that  of 
the  atmosphere.  This  has  the  effect  of  holding,  in  combination  with  the 
liquor,  air  or  gas,  which,  under  the  atmospheric  pressure  only,  would  escape. 
If  any  air  or  gas  rise  from  the  liquor  after  being  bottled,  it  causes  a  still  greater 
condensation,  and  an  increased  pressure  above  its  surface.  When  the  cork 
is  drawn  from  a  bottle  containing  liquor  of  this  kind,  the  air  fixed  in  the 
liquid,  being  released  from  the  pressure  of  the  air  which  was  condensed  under 
the  cork,  instantly  makes  its  escape,  and  rising  in  bubbles,  produces  efferves- 
cence and  froth. 


PNEUMATICS.  181 

It  sometimes  happens  that  the  united  force  of  the  air  and  gases  thus  con- 
fined in  the  bottle,  becomes  greater  than  the  cohesive  strength  of  the  parti- 
cles of  matter  composing  the  bottle ;  the  sides  of  the  bottle  in  such  cases  give 
way  or  burst. 

Those  liquors  only  froth  which  are  viscid,  glutinous,  or  thick,  like  ale,  por- 
ter, etc.,  because  they  retain  the  little  bubbles  of  air  as  they  rise ;  while  a  thin 
liquor,  like  champagne,  which  suffers  the  bubbles  to  escape  readily,  sparkles. 
.  388.  The  pressure  of  the  atmosphere  is  connected  with  the 

pressure  of  the  action  of  breathing.  The  air  enters  the  lungs,  not  because 

colSed'with  they  draw  Jt  in'  but  by  the  wei§'nt  of  the  atmosphere  forcing 
the  act  of  it  into  the  empty  spaces  formed  by  the  expansion  of  the  air- 
breathing?  cellg  Of  the  iungS-  The  air  in  turn  escapes  from  the  lungs  by 
means  of  its  elasticity ;  the  lungs,  by  muscular  action,  compress  the  air  con- 
tained in  them,  and  give  to  it  by  compression  a  greater  elasticity  than  the  air 
without.  By  this  excess  of  elasticity  it  is  propelled,  and  escapes  by  the 
mouth  and  nose. 

389.  It  has  been  proposed  to  take  advantage  of  the  pressure 
proposed8  con-  of  tlie  atm°sphere  for  the  construction  of  an  atmospheric  tele- 
structionofthe  graph,  or  apparatus  for  conveying  the  mails  and  other  matter 
over  great  distances  with  great  rapidity.  The  plan  proposed 
is  as  follows ; — a  long  metal  tube  is  laid  down,  the  interior 
surface  of  which  is  perfectly  smooth  and  even.  A  piston  is  fitted  to  the  tube 
in  such  a  manner  as  to  move  freely  in  it  and  yet  be  air-tight.  To  one  side 
of  this  piston  the  matter  to  be  moved,  made  up  in  the  form  of  a  cylindrical 
bundle,  is  attached.  A  partial  vacuum  is  then  made  in  the  tube  before  the 
piston,  by  means  of  large  air-pumps,  worked  by  steam-power,  located  at  the 
further  end  of  the  tube,  when  the  pressure  of  the  atmosphere  on  the  other 
side  of  the  piston  impels  it  forward  through  the  whole  length  of  the  exhausted 
tube.  It  has  been  estimated  that  a  piston,  drawing  after  it  a  considerable 
weight  of  matter,  could  in  this  way  be  forced  through  a  tube  at  the  rate  of 
600  miles  per  hour. 

390.  The  pressure  of  the  atmosphere  is  taken  advantage  of  in  the  con- 
struction of  a  great  variety  of  machines  for  raising  water ;  the  most  important 
and  familiar  of  which  is  the  common,  or  suction  pump. 

Describe  the  The  common,  or  suction  pump,  consists 
fhestrcom0mo°n  of>  a  hollow  cylinder,  or  barrel,  open  at  both 
pump.  ends,  in  which  is  worked  a  movable  piston, 

which  fits  the  bore  of  the  cylinder  exactly,  and  is  air-tight. 
The  pump  is  further  provided  with  two  valves,  one  of 
which  is  placed  in  the  piston,  and  moves  with  it,  while 
the  other  is  fixed  in  the  lower  part  of  the  pump-barrel. 
These  valves  are  termed  boxes. 

Fig.  173  represents  the  construction  of  the  common  pump.  The  body  con- 
sists of  a  cylinder,  or  barrel,  5,  the  lower  part  of  which,  called  the  suction- 


182 


WELLS'S  NATURAL  PHILOSOPHY. 


pipe,  descends  into  the  water  which  it.  is  designed  to  FIG.  173. 

raise.     In  the  barrel  works  a  piston  containing  a  valve, 

2?,  opening  upward.     A  similar  valve,  g,  is  fixed  in  the 

boay  of  the  pump,  at  the  top  of  the  suction-pipe.     S  is 

a  spout  from  which  the  water  raised  by  the  working  of 

the  piston  is  discharged. 

The  operation  of  the  pump  in  raising  water  is  as  fol- 
lows ; — when  the  piston  is  raised  from  the  bottom  of  the 
cylinder,  the  air  above  it  is  drawn  up,  leaving  a  vacuum 
below  the  piston  ;  the  water  in  the  well  then  rushes  up 
through  the  valve  g,  and  fills  the  cylinder  ;  the  piston  is 
then  forced  down,  shutting  the  valve,  g,  and  causing  the 
water  to  rise  through  the  piston-valve,  p ;  the  piston  is 
then  raised,  closing  its  valve,  and  raising  the  water 
above  it,  which  flows  out  of  the  spout,  S. 

391.  Water  rises  in  a  pump 

Why    does          <  .  v  i. ., .     •*.  ,      r  ••»-.•  s          .      ,        , 

water  rise  in  a    simply  and    entirely  by  the 

common  pump?  •* 

pressure    of   the    atmosphere 
(15  pounds  on  every  square  inch),  which 
pushes  it  up  into  the  void,  or  vacuum  left  by  the  up- 
drawn  piston. 

3.92.  The  common,   or  suction  pump,  can 


To  what  height 
will  water  rise 
in  the  common 


not  raise  water  beyond  the  point  of  height  at 


pump?  which  the  column  of  water  in  the  pump  tube 

is  exactly  balanced  by  the  weight  of  the  atmosphere.     The 
utmost  limit  of  this  does  not  exceed  34  feet. 

The  height  to  which  water  is  thus  forced  up  in  a  pump  is  simply  a  question 
of  balance  ;  15  pounds'  pressure  of  the  atmosphere  can  support  only  15  pounds' 
weight  of  water ;  and  a  column  of  water,  one  inch  square  and  34  feet  high, 
will  weigh  15  pounds.  As  the  pressure  of  the  atmosphere  is  subject  to  va- 
riations, and  as  the  mechanism  of  the  pump  is  never  absolutely  perfect,  the 
length  of  the  pipe  through  which  water  is  to  be  elevated  ought  never  to 
exceed  in  practice  30  feet  above  the  level  of  the  water  in  the  well,  or  reser- 
voir. 

what  is  a  ^93.  A  valve,  in  general,  is  a  contrivance  by 
vaive?  which  water  or  other  fluid,  flowing  through  a 
tube  or  aperture,  is  allowed  free  passage  in  one  direction, 
but  is  stopped  in  the  other.  Its  structure  is  such,  that, 
while  the  pressure  of  fluid  on  one  side  has  a  tendency  to 
close  it,  the  pressure  on  the  other  side  has  a  tendency  to 
open  it. 


PNEUMATICS. 


183 


Figs.  1*74,  175,  and  176,  represent  the  various  forms  of  valves  used  in 
pumps,  water-engines,  etc. 

FIG.  174.  FIG.  175.  FIG.  176. 


FIG.  177. 


394.  When  it  is  desired  to  raise  water  to  a  greater  height  than  34  feet,  a 
modification  of  the  pump,  called  the  forcing-pump,  is  employed. 

what  is  a  Tae  Forcing-Pump  is  an  apparatus  which 
Forcing-Pump?  rajses  water  from  a  reservoir,  on  the  principle  of 
the  suction-pump,  and  then,  by  the  pressure  of  the  piston 
on  the  water,  elevates  it  to  any  required  height.  . 

Fig.  177  represents  the  principle  of  the  construction 
of  the  forcing-pump.  There  is  no  valve  in  the  pis- 
ton c  (Fig.  177),  but  the  water  raised  through  the  suc- 
tion-pipe a,  and  the  valve  g,  by  the  elevation  of  the 
piston,  is  forced  by  each  depression  of  the  piston  up 
through  the  pipe  e  e,  which  is  furnished  with  a  valve  to 
prevent  the  return  of  the  liquid. 

The  forcing- purnp,  as  constructed  in  Fig.  177,  ejects 
the  water  only  at  each  stroke  of  the  piston,  in  the 
manner  of  a  syringe.  When  it  is  desired  to  make  the 
flow  of  the  water  continuous 
as  in  a  fire-engine,  an  air, 
chamber  is  added  to  the 
force-pump,  as  is  represented 
at  A,  Fig.  178.  The  water 
then,  instead  of  immediately 

passing  off  through  the  discharging-pipe,  partially 

fills  the  air  vessel,  and  by  the  action  of  the  piston 

in  the  pump,  compresses  the  air  contained  in  it. 

The  elasticity  of  the  air,  thus  compressed,  being  in- 
creased, it  reacts  upon  the  water,  and  forces  its  ascent 

in  the  discharge,  or  force-pipe.    When  the  air  in  the 

chamber  is  condensed  into  half  its  original  bulk,  it 

will  act  upon  the  surface  of  the  water  with  double 

the  atmospheric  pressure,  while  the  water  in  the 

force-pipe,  being  subject  to  only  one  atmospheric 

pressure,  there  will  be  an  unrestricted  force,  press- 


FIG.  178. 


184  WELLS'S  NATURAL   PHILOSOPHY. 

ing  the  water  up,  equal  to  one  atmosphere:  consequently,  a  column  of  water 
will  be  sustained,  or  projected  to  a  height  of  34  feet.  When  the  air  is  con- 
densed into  one  third  of  its  bulk,  its  elastic  force  will  be  increased  three- 
fold, and  it  will  then  not  only  counterbalance  the  ordinary  atmospheric 
pressure,  but  will  force  the  water  upward  with  a  pressure  equal  to  two  at- 
mospheres, or  64  feet,  and  so  on.  The  ordinary  lire-engine  is  simply  a  conve- 
nient arrangement  of  two  forcing-pumps,  furnished  with  a  strong  air-chamber, 
and  which  are  worked  successively  by  the  elevation  and  depression  of  two 
long  levers  called  brakes. 

what  is  a  395.  The  Syphon  is  an  apparatus  by  which 
syphon?  a  jjqujd  can  be  transferred  from  one  vessel  to 
another  without  inverting,  or  otherwise  disturbing  the 
position  of  the  vessel  from  which  the  liquid  is  to  be  re- 
moved. 

In  its  simplest  form,  the  syphon  consists  of  a  bent  FIG.  1  < 

tube,  ABC,  Fig.  179,  having  one  of  its  branches 
longer  than  the  other.  If  we  immerse  the  short  arm 
in  a  vessel  of  water,  and  by  applying  the  mouth  to 
the  long  arm,  as  at  C,  exhaust  the  air  in  the  tube, 
the  water  will  be  pressed  over  by  atmospheric  pres- 
sure, and  continue  to  flow  so  long  as  the  end  of  the 
lower  arm  is  below  the  level  of  the  water  in  the  vessel 
U  on  h  t  "^ie  ac^^on  °f  *he  syphon  is  readily 
principle  does  explained:  the  column  of  h'quid  in 
the  syphon  act?  the  longer  arm>  aud  that  reaching  in 

the  shorter  arm  from  the  top  of  the  curve  or  bend  to  the  surface  of  the  liquid 
i  i  the  vessel,  have  both  a  tendency  to  obey  the  attraction  of  gravity  and  fall 
o  it  of  the  tube.  This  tendency  is  opposed,  however,  on  both  sides,  by 
atmospheric  pressure,  acting  on  one  side  at  the  opening  C,  and  upon  the 
other  upon  the  surface  of  the  liquid  in  the  vessel,  thus  preventing,  in  the 
interior  of  the  tube,  the  formation  of  a  vacuum,  which  would  take  place  at 
tha  curve,  if  the  two  columns  ran  down  on  both  sides.  But  the  column 
on  one  side  being  longer  than  upon  the  other,  the  weight  of  the  long 
column  overbalances  the  short  one,  and  determines  the  direction  of  the 
flow;  and  in  proportion  as  the  liquid  escapes  from  the  long  arm,  a  fresh 
portion  is  forced  into  the  short  arm  on  the  other  side  by  the  pressure  of 
the  air.  The  syphon  is,  therefore,  kept  full  by  the  pressure  of  the  atmos- 
phere, and  kept  running  by  the  irregularity  of  the  lengths  of  the  columns  in 
its  branches. 

A  suction-tube  is  sometimes  attached  to  the  syphon  to  make  it  more  use- 
ful and  efficient,  as  is  represented  in  Fig.  180.  By  this  means  we  may  fill 
the  whole  syphon  without  the  h'quid  entering  the  mouth,  by  sucking  at 
the  end  of  the  suction-tube,  and  temporarily  closing  the  end  of  the  longer 
arm. 

In  order  that  the  discharge  of  a  liquid  by  means  of  the  syphon  should  be 


PNEUMATICS. 


185 


perfectly  constant,  it  is  nee-         FIG.  180.  PIG.  181. 

essaiy  that  the    difference 

of  lengths  of  the  columns  of 

liquid     in     both     branches 

should       be       immovable. 

This  may   be   effected    by 

connecting  the  syphon  with 

ka  float  and  pulley,    as  is 

represented  in  Fig.  181. 

The  curi- 
P^xplam       the 

phenomenon  OUS    pheno- 

of  intermitting        menon       of 
springs. 

intermitting 

springs  may  be    explained 
upon  the  principle    of  the 
syphon.     These  springs  run 
for  a  time   and   then  stop 
altogether,  and  after  a  time 
run  again,  arid   then   stop. 
If  we  suppose  a   reservoir 
in  the  interior  of  a  hill  or 
mountain,   with  a  syphon- 
like  channel  running  from  it,  as 
in  Fig.  182,  then  as  soon  as  the 
water  collecting  in  the  reservoir 
rises  to  the  height  shown  by  the 
dotted  line,  the  stream  will  be- 
gin to  flow,  and  continue  flow- 
ing till  the  reservoir  is  nearly 
emptied.     Again,    after  an  in- 
terval long   enough  to  fill  the 
reservoir  to  the  required  height, 
it  will  again  flow,  and  so  on. 

when  will  a        ^96.  If  a  solid  substance  have   the  same 
density  as  atmospheric  air,  it  will,  when  im- 
air?  mersed  in  air,  lose  its  entire  weight,  and  will 

remain  suspended  in  it  in  any  position  in  which  it  may 
be  placed. 

When    wffl    a  397'    If  a  S°lid  b°dv>  blllk  f°r  bul1^  ^6  ^iQT 

theli/?56  in     than  atmospheric  air,  it  is  pressed  upward  by 
the  surrounding  particles  of  air,  and  rises,  upon 
the  same  principle  as  a  cork  rises  from  the  bottom  of  a 
vessel  of  water.    (See  §  85.) 


186  WELLS'S   NATURAL  PHILOSOPHY. 

As  the  density  of  the  air  continually  diminishes  as  we 
ascend  fr°m  the  surface  of  the  earth,  it  is  evident  that  such 
ing  body  re-  a  body,  as  it  goes  up,  will  finally  attain  a  height  where  the  air 
main  station-  wiu  ^Q  ^  ^mQ  density  ftg  itself>  and  at  such  a  point  the 

body  wDl  remain  stationary.  Upon  this  principle  clouds,  at 
different  times,  float  at  different  degrees  of  elevation. 

It  is  also  upon  these  principles  that  aerostation,  or  the  art  of  navigating  the 
air,  depends. 

what  are  Bai-        398.    Balloons  are  machines  which   ascend 
loons?         through  the  atmosphere,  and  float  at  a  certain 
height,  in  virtue  of  being  filled  with  a  gas  or  air  lighter 
than  the  same  bulk  of  atmospheric  air. 

Balloons  are  of  two  kinds.     MONTGOLFIER, 

What  are  the 

two   yarieties    or  rarefied  air  balloons,  and  HYDROGEN  GAS 

of  balloons  ? 

balloons.  The  first  are  filled  with  common 
air  rarefied  by  heat,  and  thus  made  lighter  than  the 
surrounding  atmosphere ;  while  the  second  are  filled 
with  hydrogen,  a  gas  about  fourteen  times  lighter  than 
air. 

The  rarefied  air-balloon  was  invented  by  Montgolfier,  a 
Montgoifier,  or  French  gentleman,  in  1782,  who  first  filled  a  paper  bag  with 
lotmfiedairbal"  h«ated  air,  and  allowed  it  to  pass  up  a  chimney.  He  after- 
ward constructed  balloons  of  silk,  of  a  spherical  shape,  with 
an  aperture  formed  in  the  lower  surface.  Beneath  this  opening  a  light  wire 
basket  was  suspended,  containing  burning  material.  The  hot  air  arising  from 
the  burning  substances,  enters  the  aperture,  and  rendering  the  balloon  specific- 
ally lighter  than  the  air,  causes  it  to  ascend  with  considerable  velocity. 
Small  balloons  of  a  similar  character  are  frequently  made  at  the  present  day 
of  paper,  the  air  within  them  being  rarefied  by  means  of  a  sponge  soaked  in 
alcohol,  suspended  by  a  wire  beneath  the  mouth,  and  ignited. 
Describe  the  The  hydrogen  gas  balloon  consists  of  a  light  silken  bag, 
hydrogen  gas  filled  either  with  hj^drogen,  or  common  illuminating  gas.  The 
difference  between  the  specific  weight  of  either  of  these  gases 
and  common  air  is  so  great,  that  a  large  balloon  filled  with  them  possesses 
ascensional  power  sufficient  to  rise  to  great  heights,  carrying  with  it  consid- 
erable additional  weight.  The  aeronaut  can  descend  by  allowing  the  gas  to 
escaps  by  means  of  a  valve,  thereby  diminishing  the  bulk  of  the  balloon.  To 
enable  him  to  rise  again,  ballast  is  provided,  generally  consisting  of  bags  of 
sand,  by  throwing  out  which,  the  balloon  is  lightened,  and  accordingly 
rufcs. 

By  means  of  one  of  these  machines  Gay  Lussac,  an  eminent  French  chem- 
ist, ascended  in  1804,  for  the  purpose  of  making  meteorological  observations, 
to  the  great  height  of  23,000  feet. 


PNEUMATICS.  187 

399.    Air    °es     tn<3    laWS    °f  m°tion    which 


Do  the  laws  of 

motion  apply    are  common  to  all  other  material  and  ponder- 

to  air  '(  •*• 

able  substances. 

400.  -  The  momentum  of  air,  or  the  amount 
°f  f°rce  which  it  is  capable  of  exerting  upon 
bodies  opposed  to  it,  is  estimated  in  the  same 

way  as  in  the  case  of  solids,  viz.,  by  multiplying  its  weight 

by  its  velocity. 

The  momentum  of  air  is  usefully  employed  as  a  mechanical 
trations  of  the  agent  in  imparting  motion  to  wind-mills  and  to  ships.  Its 
momentum  of  most  striking  effects  are  seen  in  the  force  of  wind,  which  oc- 
casionally, in  hurricanes  and  tornadoes,  acts  with  fearful 
power,  prostrating  trees  and  buildings.  Such  results  are  caused  by  the  mo- 
mentum of  the  air  being  greater  than  the  force  by  which  a  building,  or  a  tree 
is  fastened  to  the  earth. 

401.  Any  force  acting  suddenly  upon  the  air  from  a  center, 
What      causes  J  A  i         ±-c  i    -n 

the    rings    of      imparts  to  it  a  rotary  movement.     A  very  beautiful  illustra- 

tion  of  tnis  is  seen  in  tne  rinSs  of  smoke  which  are  produced 
dis-     by  the  mouth  of  a  skilful  tobacco-smoker,  and  frequently  also 

uP°n  a  much  lar£er  scale  bv  the  discharSe  of  cannon,  on  a 

still  day.  In  these  cases  a  portion 
of  air  acted  upon  suddenly  from  a  center  is  caused 
to  rotate,  and  the  particles  of  smoke  render  the  mo- 
tion  visible.  The  whole  circumference  of  each 
circle  is  in  a  state  of  rapid  rotation,  as  is  shown  by 
the  arrows  in  Fig.  183.  The  rapid  rotation  in 
short,  confines  the  smoke  within  the  narrow  limits  of  a  circle,  and  causes  the 
rings  to  be  well  defined. 

PRACTICAL   PROBLEMS   IN   PNEUMATICS. 

1.  If  100  cubic  inches  of  air  weigh  31  grains,  what  will  he  the  weight  of  one  cubic  foot  ? 

2.  If  the  pressure  of  the  atmosphere  be  15  pounds  upon  a  square  inch,  what  pressure 
will  the  body  of  an  animal  sustain,  whose  superficial  surface  is  forty  square  feet  ? 

3.  When  the  elevation  of  the  mercury  in  the  barometer  is  28  inches,  what  will  be  the 
height  of  a  column  of  water  supported  by  the  pressure  of  the  atmosphere  ? 

Solution:  Column  of  mercury  supported  by  the  atmosphere  =  28  inches.  Mercury 
being  13J  times  heavier  than  water,  the  column  of  water  supported  by  the  atmosphere  — 
13^x28=31  feet.  *S8J*  ';' 

4.  When  the  elevation  of  the  mercury  in  the  barometer  is  30  inches,  what  will  be  the 
height  of  a  column  of  water  supported  by  the  atmosphere?  » 

5.  To  what  height  may  water  be  raised  by  a  common  pump,  at  a  place  where  the  ba- 
rometer stands  at  24  inches  ? 

6  If  a  cubic  inch  of  air  weighs  .30  of  a  grain,  what  weight  of  air  will  a  vessel  whose 
capacity  is  60  cubic  inches,  contain  ? 


CHAPTER    XI. 

ACOUSTICS. 

what  ig  the  4°2'  AcouSTICS  is  tllat  department  of  phys- 
science  of  ical  science  which  treats  of  the  nature,  phe- 

Acoustics?  ,    ,  ,,  ,         _ 

nomena,  and  laws  of  sound.     It  also  includes 
the  theory  of  musical  concord  or  harmony. 

what  is  sound?  ^^'  ®oun&  *s  t^ie  sensation  produced  on  the 
organs  of  hearing,  when  any  sudden  shock  or 
impulse,  causing  vibrations,  is  given  to  the  air,  or  any 
other  body,  which  is  in  contact,  directly  or  indirectly,  with 
the  ear. 

under whatcir-  404.  When  an  elastic  body  is  disturbed  at 
v?b?ato?ymove-  anv  point,  its  particles  execute  a  series  of  vi- 
ments arise?  bratory  movements,  and  gradually  return  to  a 
position  of  rest. 

Thus  when  a  glass  tumbler  is  struck  by  a  hard  body,  a  tremulous  agitation 
is  transmitted  to  its  entire  mass,  which  movement  gradually  diminishes  in 
force  until  it  finally  ceases.  Such  movements  in  matter  are  termed  vibra- 
tions, and  when  communicated  to  the  ear  produce  a  sensation  of  sound. 

The  nature  of  these  vibratory  movements  may  be  illustrated  by  noticing 
the  visible  motions  which  occur  on  striking  or  twitching  a  tightly  extended 
cord,  or  wire.  Suppose  such  a  cord,  repre-  Fir} 

sented  by  the  central  line  in  Fig.  184  to  be 
forcibly  drawn  out  to  A,  and  let  go ;  it 
would  immediately  recover  its  original  posi- 
tion by  virtue  of  its  elasticity ;  but  when  it 
reached  the  central  point,  it  would  have  ac- 
quired so  much  momentum  as  would  cause 
it  to  pass  onward  to  a ;  thence  it  would  vi- 
brate back  in  the  same  manner  to  B,  and  back  again  to  b,  the  extent  of  ita 
vibration  being  gradually  diminished  by  the  resistance  of  the  air,  so  that  it 
would  at  length  return  to  a  state  of  rest. 

Describe  the  In  vibratory  movements  of  this  kind  all  the  separate  par- 
naRire  of  a^sta-  ^icles  come  into  motion  at  the  same  time,  simultaneously  pass 
tion,  the  point  of  equilibrium,  or  rest,  simultaneously  reach  the 

maximum  of  their  vibration,  and  simultaneously  begin  their  retrograde  mo- 
tion. Such  vibrations  are  therefore  called  stationary,  or  fixed  vibrationa 


ACOUSTICS. 


189 


If!  however,  the  motions  of  the  vibrating  body  are  of  such  a 
Describe     the  J      .  , 

nature    of    a      character  that  the  agitation  proceeds  from  one  particle  to  an- 

progressive  vi-  other,  so  that  each  makes  the  same  vibration,  or  oscillation, 
bration. 

as  the  preceding  one,  with  the  sole  exception  of  the  motion 

beginning  later,  we  have  what  is  called  progressive  vibrations.  Thus  if  we 
fasten  a  cord  at  one  end,  and  move  the  other  end  up  and  down,  a  wave,  or 
progressive  vibration,  is  produced. 

As  the  clearest  conception  can  be  formed  of  vibrations  by  comparing  them 
to  the  waves  produced  by  throwing  a  stone  into  smooth  water,  the  term  un- 
dulatory,  or  wave  movement,  has  been  adopted  in  general  to  express  the 
phenomena  of  vibrations. 

403.  Daily  experience  teaches  us  that  almost  every  motion  of  bodies  in  our 
vicinity  is  accompanied  by  a  noise  perceptible  to  our  ears.  All  such  sounds 
are  the  result  of  the  vibrations  of  a  portion  of  matter,  and  the  nature  of  the  tone, 
or  sound,  depends  only  on  the  manner  in  which  these  vibrations  originate.  - 
406.  Sound-vibrations  in  solid  bodies  may  be  rendered  vis- 
ible by  many  simple  contrivances.  If  we  attach  a  ball  by 
means  of  a  string  to  a  bell,  and  strike  the  bell,  the  ball  will 
vibrate  so  long  as  the  bell  continues  to  sound.  When  a  bell 
is  sounding,  -also,  the  tremulous  motion  of  its  particles  may  be 
perceived  by  gently  touching  it  with  the  finger.  If  the  finger  is  pressed 
firmly  against  the  bell,  the  sound  is  stopped,  because  the  vibrations  are  in- 
terrupted. When  sounds  are  produced  by  drawing  the  wet  finger  around  the 
edge  of  a  glass  containing  water,  waves  will  be  seen  undulating  from  the  sides 
toward  the  center  of  the  glass. 

When  a  tuning-fork  is  struck  and  made  to  sound,  its  vibrations  FIG-  1S5. 
Are  clearly  visible,  both  branches  alternately  approaching  and  re- 
receding  from  each  other,  as  is  represented  in  Fig.  185. 

If  we  strike  a  tuning-fork,  and  then  touch  the  surface  of  mercury 
with  one  of  its  extremities,  the  surface  of  the  mercury  will  exhibit 
little  undulations  or  waves. 


How  may  the 
sound-vibra- 
tions in    solid 
bodies  be  ren- 
dered visible? 


The  most  interesting  method  of  exhibiting  the 
character  of  sound   is  by  means  of  the  so-called 


D 


Ilnw    are    the 
fio-callad  acous- 

d'uced™  pr°-     "acoustic  figures,"  which  may  be  produced  in  the 
following  manner : — Sprinkle  some  fine  sand  over  a 
square  or  round  piece  of  thin  glass  or  metal,  and  holding  the  plate 
firmly  by  means  of  a  pair  of  pincers,  draw  a  violin  bow  down  the  edge ;  the 
sand  is  put  in  motion,  and  finally  arranges  itself  along  those  parts  of  the  surface 

which  have  the  least  vi- 

FlG-  186<  bratory  motion.  By  chang- 

ing the  point  by  which 
the  plate  is  held,  or  py 
varying  the  parts  to  which 
the  violin  bow  is  applied, 
the  sand  may  be  made  to 
assume  various  interesting  figures,  as  is  represented  in  Fig.  186. 


190  WELLS'S  NATURAL  PHILOSOPHT. 


what  is  the  '          *s  ^e  usual  medium  through  which 

usual  medium     sound  is  conveyed  to  the  ear.     The  vibrating 

through  which  .  J 

sound  ispropa-     body  imparts  to  the  air  in  contact  with  it  an 
undulatory,  or  wave-like   movement,    which, 
propagating  itself  in  every  direction,  reaches  the  ear,  and 
produces  the  sensation  of  sound. 

what  are  son-  408.  Vibrating  bodies  which  are  capable  of 
jorous  bodies?  tnug  imparting  undulations  to  the  air,  are 
termed  sounding,  or  sonorous  bodies. 

The  aerial  vibrations,  or  undulations  thus  caused,  propagate  themselves 
from  the  center  of  disturbance  in  concentric  circles,  in  the  same  way  that 
waves  spread  out  upon  the  smooth  surface  of  water.  If  such  waves  of  water, 
propagated  from  a  denter,  encounter  any  obstruction,  as  a  floating  body,  they 
will  bend  their  course  round  the  sides  of  the  obstacle,  and  spread  out  obliquely 
beyond  it.  So  the  undulations  of  air,  if  interrupted  in  their  progress  by  a 
high  wall  or  other  similar  impediment,  will  be  continued  over  its  summit  and 
propagated  on  the  opposite  side  of  it. 

In  a  sound-wave  or  undulation  of  the  air,  as  in  a  wave  of  water,  there  is 
no  permanent  change  of  place  among  the  particles,  but  simply  an  agitation, 
or  tremor,  communicating  from  one  particle  to  another,  so  that  each  particle, 
like  a  pendulum  which  has  been  made  to  oscillate,  recovers  at  length  its 
original  position. 

This  motion  may  be  best  illustrated  by  comparing  it  to  the  motion  pro- 
duced by  the  wind  in  a  field  of  grain.  The  grassy  waves  travel  visibly  over 
the  field  in  the  direction  in  which  the  wind  blows  ;  but  this  appearance  of 
an  object  moving  is  only  delusive.  The  only  real  motion  is  that  of  the  heads 
of  the  grain,  each  of  which  goes  and  returns  as  the  stalk  stoops  or  recovers 
itself.  This  motion  affects  successively  a  line  of  ears  in  the  direction  of  the 
wind,  and  affects  simultaneously  all  the  ears  of  which  the  elevation  or  de- 
pression forms  one  visible  wave.  The  elevations  and  depressions  are  propa- 
gated in  a  constant  direction,  while  the  parts  with  which  the  space  is  filled 
only  vibrate  to  and  fro.  Of  exactly  such  a  nature  is  the  propagation  of  sound 
through  air. 

under    what        409.  If  no  substance  intervenes  between  the 
v&rating  body  and  the  organs  of  hearing,  no 
ounV?  hear    sensation  of  sound  can  be  produced. 

This  is  readily  proved  by  placing  a  bell,  rung  by  the  action 
of  clock-work,  beneath  the  receiver  of  an  air-pump,  and  exhausting  the  air. 
No  sound  .will  then  be  heard,  although  the  striking  of  the  tongue  upon  the 
bell,  and  the  vibration  of  the  bell  itself,  are  visible.  Now,  if  a  little  air  be 
admitted  into  the  receiver,  a  faint  sound  will  begin  to  be  heard,  and  this 
sound  will  become  gradually  louder  in  proportion  as  the  air  is  gradually  read- 
mitted, until  the  air  within  the  receiver  is  in  the  same  condition  as  that  without 


ACOUSTICS,  191 

Sound,  therefore,  cannot  be  propagated  through  a  vacuum. 

"  The  loudest  sound  on  earth,  therefore,  cannot  penetrate  beyond  the 
limits  of  our  atmosphere ;  and  in  the  same  manner,  not  the  faintest  sound 
can  reach  our  earth  from  any  of  the  other  planets.  Thus  the  most  fearful 
explosions  might  take  place  in  the  moon,  without  our  hearing  anything  of 
them.1' 

HO*T  does  the  410.  The  power  of  air  to  transmit  sound 
varies  with  its  uniformity,  its  density,  and  its 
humidity. 

"Whatever  tends  to  agitate  or  disturb  the  condition  of  the  atmosphere,  affects 
the  transmission  of  sounds.  "When  a  strong  wind  blows  from  the  hearer  to- 
ward a  sounding  body,  a  sound  often  ceases  to  be  heard  which  would  be 
audible  in  a  calm.  Falling  rain,  or  snow,  interferes  with  the  undulations  of 
sound-waves,  and  obstructs  the  transmission  of  sound. 

The  fact  that  we  hear  sounds  with  greater  distinctness  by 
hS?  drouJds  niSht  than  by  day,  may  be,  in  part,  accounted  for  by  the  cir- 
more  distinctly  cumstance,  that  the  different  layers  or  strata  of  the  atmosphere 
by  day?  are  ^ess  liable  to  variations  in  density  and  to  currents,  caused 

by  changes  of  temperature,  at  night  than  by  day.  The  air  at 
night  is  also  more  still,  from  the  suspension  of  business  and  hum  of  men. 
Many  sounds  become  perceptible  during  the  night,  which  during  the  day  are 
completely  stifled,  before  they  reach  the  ear,  by  the  din  and  discordant  noises 
of  labor,  business,  and  pleasure. 

Sound  of  any  kind  is  transmitted  to  a  greater  distance  in  cold  and  clear 
weather  than  in  warm  weather,  the  density  of  air  being  increased  by  cold 
and  diminshed  by  heat. 

On  the  top  of  high  mountains,  where  the  air  is  greatly  rare- 
lustrations  of  ^e<^»  *tte  sound  of  the  human  voice  can  be  heard  for  a  short 
the  variation  distance  only ;  and  on  the  top  of  Mont  Blanc,  the  explosion 
air?S°Und  m  of  a  P^tol  appears  no  louder  than  that  of  a  small  cracker. 
"When  persons  descend  to  any  considerable  depth  in  a  diving- 
bell,  the  air  around  them  is  compressed  by  the  weight  of  a  considerable  column 
of  water  above  them.  In  such  circumstances,  a  whisper  is  almost  as  loud  as  a 
shout  in  the  open  air;  and  when  one  speaks  with  ordinary  force  it  produces 
an  effect  so  loud  as  to  be  painful. 

is  air  neces-        411.  Air  is  not  necessary  to  the  production 
JrodJtiL  *  of    °f  sound,  although  most  sounds  are  transmitted 

by  its  vibrations.  Sound  can  be  produced  un- 
der water,  and  all  bodies  are  more  or  less  fitted,  not  only 
to  produce,  but  also  to  transmit  sounds. 

412.  Sound  is  communicated  more  rapidly 

and  more  distinctly  through  solid  bodies  than 
most  readily?  through  either  liquids  or  gases.  It  is  trans- 


192  WELLS'S   NATUKAL   PHILOSOPHY. 

mitted  by  water  near  four  times  more  rapidly  than  by  air, 
and  by  solids  about  twice  as  rapidly  as  by  water. 

If  we  strike  two  stones  together  under  water,  the  sound  will  be  as  loud  as 
if  they  had  been  struck  in  the  air. 

When  a  stick  is  held  between  the  teeth  at  one  extremity,  and  the  other  is 
placed  in  contact  with  a  table,  the  scratch  of  a  pin  on  the  table  may  be  heard 
with  great  distinctness,  though  both  ears  be  stopped. 

The  earth  often  conducts  sound,  so  as  to  render  it  sensible  to  the  ear,  when 
the  air  fails  to  do  so.  It  is  well  known  that  the  approach  of  a  troop  of  horse 
can  be  heard  at  a  distance  by  putting  the  ear  to  the  ground,  and  savages 
practice  this  method  of  ascertaining  the  approach  of  persons  from  a  great  dis- 
tance. 

The  principle  that  solids  transmit  sounds  more  perfectly  than  air,  has  been 
applied  to  the  construction  of  an  instrument  called  the  "stethoscope." 
j-j      ..        ,  The  stethoscope  consists  of  a  hollow  cylinder  of  wood,  some- 

Stethoscope.  what  resembling  in  form  a  small  trumpet.  The  wide  mouth 
is  applied  firmly  to  the  breast,  and  the  other  is  held  to  the  ear 
of  the  medical  examiner,  who  is  thus  enabled  to  hear  distinctly  the  action  of 
the  organs  of  respiration,  and  judge  whether  they  are  in  a  healthy  condition, 
or  the  reverse. 

HOW  is  the  in-  413.  Sound  decreases  in  intensity  from  the 
affectedtyTs^  center  where  it  originates,  according  to  the 
tance?  same  law  by  which  the  attraction  of  gravita- 

tion varies,  viz.,  inversely  as  the  square  of  the  distance. 
That  is  to  say,  at  double  the  distance  it  is  only  one  fourth 
part  as  strong  ;  at  three  times  the  distance,  one  ninth,  and 
so  on. 

This  law  applies  with  its  full  force  only  when  no  opposing  currents  of  air, 
or  other  obstacles,  interfere  with  the  wave  movements,  or  undulations.  By 
confining  the  sound  undulations  in  tubes,  which  prevent  their  spreading,  the 
force  of  sound  diminishes  much  less  rapidly.  It  will,  therefore,  under  such 
circumstances,  extend  to  much  greater  distances.  This  principle  is  taken  ad- 
vantage of  in  the  construction  of  speaking-trumpets. 

Sounds  can  generally  be  heard,  especially  on  a  calm  day,  at 
a  greater  distance  upon  water  than  upon  land.     The  plane 


tinctly    upon      surface  of  water,  as  a  smooth  wall,  prevents  the  lateral  spread- 
land  ?  ing  and  dispersion  of  the  sound-waves,  although  on  only  one 

side.  The  air  over  water,  owing  to  the  presence  of  moist- 
ure, is  also  generally  more  dense,  and  the  density  more  uniform  than 
over  the  land.  Water,  in  addition,  is  a  better  conductor  of  sound  than 
the  earth. 

The  transmission  of  sound  from  one  apartment  to  another  may  be  prevented 
by  filling  up  the  spaces  between  fhe  partition  walls  with  shavings,  or  any 
porous  substances.  The  number  of  media  through  which  the  sound  must 


ACOUSTICS.  193 

pass  is  thus  greatly  increased,  and  every  change  of  medium  diminishes  tho 
strength  of  sound-waves. 

414.  The  velocity  of  the  sound  undulations 

What  law  gov-        .  .--•'  . 

ems  the  veioc-     is   Tin  norm,  passing;   over   equal  intervals    in 

ily  of  sound?  _      . 

equal  times. 

The  softest  whisper,  therefore,  flies  as  fast  as  the  loudest  thunder. 

with  what  ve-  415.  Sounds  of  every  kind  travel,  when  the 
Si  travel?  temperature  is  at  62°  Fahrenheit's  thermom- 
eter, at  a  rate  of  1,120  feet  per  second,  or 
about  13  miles  per  minute,  or  765  miles  per  hour.  The 
velocity  of  sound  increases  or  diminishes  at  the  rate  of  13 
inches  for  every  variation  of  a  degree  in  temperature  above 
or  below  the  temperature  of  62°  Fahrenheit. 

Why  do  we  see         When  a  gun  is  fired  at  some  distance,  we  see  the  flash  a 

the  flash  of  a      considerable  time  before  we  hear  the  report,  for  the  reason 

hear  ^report?    that  W*  travels  much  faster  than  sound.     Light  would  go 

round  the  earth  480  times  while  sound  was  traveling  13  miles. 

A  knowledge  of  these  circumstances  is  taken  advantage  of  for  the  measure- 
ment of  distances. 

How  may  a  Thus,  suppose  a  flash  of  lightning  to  be  perceived,  and  on 
thoTefiwIt  of  coanting  the  seconds  that  elapse  before  the  thunder  is  heard, 
Bound  be  ap-  we  find  them  to  amount  to  20;  then  as  sound  moves  1,120 
measurement"  feet  iri  a  second»  Jt  wiu  follow  that  the  thunder-cloud  must  be 
of  distances  ?  distant  1,120X20  =  22,400  feet. 

When  a  long  column  of  soldiers  are  marching  to  a  measure  beaten  on  tho 
drums  which  precede  them,  we  may  observe  an  undulatory  motion  transmit- 
ted from  the  drummers  through  the  whole  column,  those  in  the  rear  stepping 
a  little  later  than  those  which  precede  them.  The  reason  of  this  is,  that  each 
rank  steps,  not  when  the  sound  is  actually  made,  but  when  in  its  progress 
down  the  column  at  the  rate  of  1,120  feet  in  a  second  of  time,  it  reaches  their 
ears.  Those  who  are  near  the  music  hear  it  first,  while  those  at  the  end  of 
the  column  must  wait  until  it  has  traveled  to  their  ears  at  the  above  rate. 
Explain  the  416.  If  two  waves  of  water,  advancing  from  opposite  direc- 

fh^ilSeJencl  tion8'  meet  in  such  &  way  that  their  Points  of  elevation  coin- 
of  sound.  cide,  a  wave  of  double  the  height  of  the  single  one  will  be 

formed  at  the  point  of  interception ;  or  if  two  wave  depressions  on  the  sur-, 
face  of  water  meet,  a  depression  of  double  depth  will  be  produced.  If,  how- 
ever, the  two  waves  come  into  contact  in  such  a  manner  that  an  elevation  of 
one  wave  coincides  with  the  depression  of  another,  both  will  be  destroyed.; 
Such  a  result  is  termed  an  interference  of  waves.  In  the  same  manner  when, 
two  series  of  sound  undulations,  propagated  from  different  sounding  bodies, 
intersect  each  other,  a  like  phenomena  of*nterference  is  produced — the  two 
undulations  destroy  each  other,  and  silence  is  produced.  ...ur 

9 


194 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  187.  Let  a  6  and  c  d,  Fig.  187,  represent  two  se- 

ries of  sound  undulations,  advancing  in  such 
a  manner  as  to  cause  the  elevation  of  one  at  e 
to  correspond  with  the  depression  of  the  other 
at/;  then  if  both  are  equal  in  intensity,  they 
will  neutralize  each  other,  and  an  instant  of  si- 
lence will  be  produced.  This  fact  may  be  very 
prettily  illustrated  by  holding  a  common  tuning- 
fork,  after  it  has  been  put  in  vibration,  over  the 
mouth  of  a  cylindrical  glass  vessel,  as  A,  Fig.  188.  The  air  contained  within 


FIG.  188. 


the  vessel  will  assume  sonorous  vibrations,  and  a 
tone  will  be  produced.  If  now  a  second  glass 
cylinder  be  held  in  the  position  B,  at  right  angles 
to  A,  the  musical  tone  previously  heard  will  cease; 
but  if  either  cylinder  be  removed,  the  sound  will 
be  renewed  again  in  the  other.  In  this  curious 
experiment,  the  silence  arises  from  the  interference 
of  the  two  sounds. 

Another  example  of  this  phenomena  may  be 
produced  by  the  tuning-fork  alone.  If  this  instrument,  after  being  put  into  vi- 
bration, be  held  at  a  great  distance  from  the  ear,  and  slowly  turned  round  its 
axis,  a  position  of  the  two  branches  will  be  found  at  which  the  sound  will 
become  inaudible.  This  position  will  correspond  to  the  points  of  interference 
of  the  two  systems  of  undulations  propagated  from  the  two  branches,  or 
prongs  of  the  fork. 

417.  The  loudness  of  a  sound,  or  its  degree 
intensity,  depends  on  the  force  with  which 
e  vibrations  of  a  sounding  body  are  made. 


ihels°ounde8Sde- 


What  are  nm 
sical  sounds  ? 


SECTION    I. 

MUSICAL     SOUNDS. 

418.  All  vibrations  of  sonorous  bodies  which 
are  uniform,  regular,   and  sufficiently  rapid, 

produce  agreeable,  or  musical  sounds. 

419.  What  constitutes  the  particular  differ- 
ence  between  a  noise  and  a  musical  sound  is 

and  a  noise?  not  certainly  known.  A  noise,  however,  is 
supposed  to  be  occasioned  by  impulses  communicated  ir- 
regularly to  the  ear  ;  but  in  a  musical  sound  the  vibra- 
tions of  the  sonorous  bodyfand  consequently  the  undula- 
tions of  the  air,  must  be  all  exactly  similar  in  duration 


MUSICAL   SOUNDS.  195 

and  intensity,  and  must  recur  after  exactly  equal  inter- 
vals of  time. 

what  is  meant  420.  If  the  sound  impulses  be  repeated  at 
verv  snort  intervals,  the  ear  is  unable  to  at- 
tend to  them  individually,  but  hears  them  as 
a  continued  sound,  which  is  uniform,  or  has  what  is  called 
a  tone  or  pitch,  if  the  impulses  be  similar  and  at  equal 
intervals. 

What   e    eri  421'  The  nature  °^  musical  sounds,  and  indeed  of  all  sounds, 

ment  illustrates    may  be  illustrated  by  the  following  experiment :  If  we  take 

musfcaUound*?     a  thin  elastic  Plate  of  metal>  a  few  inches  in  length,  firmly 
fixed  at  one   end,  and  free  at  the  other,   and  cause  it  to 
vibrate,  it  will  be  found  to  emit  a  clear,  musical  sound,  having  a  certain 
tone. 

If  the  plate  be  gradually  lengthened,  it  yields  tones,  or  notes,  of  different 
characters,  until  finally  the  vibrations  become^so  slow  that  the  eye  can  follow 
them  without  difficulty,  and  all  sound  ceases. 

422.  When  the  impulses,  or  vibrations,  are  few  in  number 
When  is  a  tone 
grave  or  sharp  ?    ma  given  time,  the  tone  is  said  to  be  grave ;  when  they  are 

many,  the  tone  is  said  to  be  sharp.  Musical  sounds  are  spoken 
of  as  notes,  or  as  high  and  low.  Of  two  notes,  the  higher  is  that  which  arises 
from  more  rapid,  and  the  lower  from  slower  vibrations. 

Beside  this,  sounds  differ  in  their  quality.  The  same  musical  note,  pro- 
duced with  the  same  degree  of  loudness,  and  by  the  same  number  of  vibra- 
tions in  the  flute,  the  clarionet,  the  piano,  and  the  human  voice,  is  in  each 
instance  peculiar  and  wholly  different.  "Why  this  is  we  are  unable  to  say. 
The  French  call  this  property,  by  which  one  sound  is  distinguished  from  an* 
other,  the  timbre. 

Is  there   an  To  Produce  an7  sound  whatever  it  is  necessary  that  a  cer- 

limit  to  the  tain  number  of  vibrations  should  be  made  in  a  certain  time. 
bStioM  requi"  If  the  number  produced  in  a  second  falls  below  a  certain  rate, 
site  to  produce  no  sound  sensation  will  be  made  upon  the  ear.  It  is  believed 
that  the  ear  can  distinguish  a  sound  caused  by  fifteen  vibra- 
tions in  a  second,  and  can  also  continue  to  hear  though  the  number  reaches 
48, 000  per  second.  Trained  and  sensitive  ears  are  said  to  be  able  to  exceed 
these  limits. 

when  are  two  423.  Two  musical  notes  are  said  to  be  in 
unison  when  the  vibrations  which  cause  them 
are  performed  in  equal  times. 

^^*  When  one  note  makes  twice  the  number 
of  vibrations  in  a  given  time  that  another  makes, 
it  is  said  to  be  its  octave.     The  relation,  or  interval  which 


196  WELLS'S   NATURAL  PHILOSOPHY. 

exists  between  two  sounds,  is  the  proportion  between 
their  respective  numbers  of  vibrations. 
what  is  a  425.  A  combination  of  harmonious  sounds 

ord,  etc.?  jg  termed  a  musical  chord;  a  succession  of  har- 
monious notes,  a  melody;  and  a  succession  of  chords,  har- 
mony. 

A  melody  can  be  performed,  or  executed  by  a  single  voice ;  a  harmony 
requires  two  or  more  voices  at  the  same  time. 

Define  concord  426.  When  two  tones,  or  notes,  sounded  to- 
and  discord.  gether  produce  an  agreeable  effect  on  the  ear, 
their  combination  is  called  a  musical  concord ;  when  the 
effect  is  disagreeable,  it  is  called  a  discord. 

Explain  what  427>  Suppose  we  nave  a  stretched  string,  as  a  wire  or  a 
is  meant  by  the  piece  of  catgut,  such  as  is  used  for  stringed  instruments:  now 
of  Susie"0*16  the  number  of  *ibrations  which  such  a  cord  will  make  in  a 
given  time,  are  inversely  as  its  length ;  that  is,  if  the  whole 
cord  makes  a  given  number  of  vibrations  in  one  second,  as  100,  on  shortening 
it  one  half  it  will  make  twice  as  many,  or  200,  and  this  will  yield  a  note  ex- 
actly an  octave  higher  than  the  former  one.  If  we  reduce  its  length  three- 
fourths,  it  will  make  four  times  as  many  vibrations  as  at  first,  and  yield  a 
note  two  octaves  higher. 

Suppose  the  stretched  string,  or  wire,  to  be  32  inches  in  length.  When 
this  is  struck  it  will  vibrate  a  certain  number  of  times  in  a  second,  and  give 
what  is  called  a  key-note.  Reduce  the  string  one  half,  and  we  have  the  oc- 
tave of  that  note.  But  between  the  key-note  and  its  octave  there  is  a  natu- 
ral gradation  by  intervals  in  the  pitch  of  the  tone,  which  heard  in  succession 
are  harmonious,  the  octave,  as  its  name  implies,  being  the  eighth  pitch  of 
tone,  or  eighth  successive  note  ascending  from  the  key-note. 

These  eight  notes,  or  intervals  in  the  pitch  of  tone  between  the  key-note 
and  its  octave,  constitute  what  is  called  the  gamut,  or  diatonic  scale  of  music, 
because  they  are  the  steps  by  which  the  tone  naturally  ascends  from  any  note 
to  the  corresponding  tone  above,  produced  by  vibrations  twice  as  rapid. 
These  several  notes  are  distinguished  both  by  letters  and  names.  They  are : 

C,    D,    E,    F,    G,    A,    B,    C; 
Or — do,  re,  me,  fa,  sol,    la,   si,    do. 

How  are  the  Thev  may  also  be  distinguished  b7  numbers  indicating  the 
notes  of  the  length  of  the  strings  and  the  number  of  vibrations  required 
scale  indicated?  to  pro(juce  them.  Thus,  the  length  of  the  string  producing 
the  primary,  or  key-note,  being  32  inches,  the  lengths  of  the  strings  to  produce 
the  tones  in  the  entire  scale  are — 

32,  30,  2T,  2,4,  21,  20,  18,  16; 

or,  supposing  that  whatever  be  the  number  of  vibrations  per  second  necessary 
to  produce  the  first  note  in  the  scale,  C,  we  agree  to  represent  it  by  unity, 


KEFLECTION   OF   SOUND.  197 

or  1  ;  then  the  numbers  necessary  to  produce  the  other  seven  notes  of  the 
octave  will  be  as  follows  : 

Name  of  note    .....  C,  D,  E,  F,  G,  A,  B,  C. 

Number  of  vibrations  .  1,    {j,   £,   f  ,  f  ,  |,    V,  2. 

However  far  this  musical  scale  may  be  extended,  it  will  still  be  found  but 
a  repetition  of  similar  octaves.  The  vibrations  of  a  column  of  air  in  a  pipe 
may  be  regarded  as  obeying  the  same  general  laws  ;  notes  are  naturally  higher 
in  proportion  to  the  short  uess  of  the  pipes. 

The  same  note  produced  on  any  musical  instrument  is  due 
note  in  any  in-      to  the  same  number  of  vibrations  per  second.     Thus,  a  note 


the 
same  manner?      ond,  is  also  produced  in  the  flute  by  a  column  of  air  vibrat- 

ing the  same  number  of  times  in  a  second,  and  also  in  the  hu- 
man voice  by  two  chords  contained  in  the  upper  part  of  the  wind-pipe,  also 
vibrating  the  same  number  of  times  in  a  second. 

It  has  been  already  stated  that  the  number  of  vibrations  of  a  cord  are  in- 
versely as  its  length  ;  the  number  also  increases  as  the  square  root  of  the 
force  which  stretches  it.  Thus  an  octave  is  given  by  the  same  length  of  string 
when  stretched  four  times  as  strongly. 


SECTION    II. 

REFLECTION    OF    SOUND. 

428.  When  waves  of  sound  strike  asrainst 

What  is  meant  ° 

any  surface  tolerably  smooth,  they  are 

reflected,  or  rebound  from  that  surface,  and 
the  angle  of  reflection  is  equal  to  the  angle  of  incidence. 

This  law  governing  the  reflection  of  sound  is  the  same  as  that  which  gov- 
erns the  reflection  of  all  elastic  bodies,  and  also,  as  will  be  shown  hereafter, 
the  imponderable  agents,  heat  and  light. 

what  i«  an  429.  An  ECHO  is  a  repetition  of  sound  caused 
«*«?  ky  the  reflection  of  the  sound  waves,  or  undu- 
lations, from  a  surface  fitted  for  the  purpose,  as  the  side 
of  a  house,  a  wall,  hill,  etc.  ;  the  sound,  after  its  first  pro- 
duction, returning  to  the  ear  at  distinct  intervals  of  time. 

Thus  if  a  body  placed  at  a  certain  distance  from  a  hearer  produces  a  sound, 
this  sound  would  be  heard  first  by  means  of  the  sonorous  undulations  which 
produced  it,  proceeding  directly  and  uninterruptedly  from  the  sonorous  body 
to  the  hearer,  and  afterward  by  sonorous  undulations  which,  after  striking  on 
reflecting  surfaces,  return  to  the  ear.  These  last  constitute  an  echo. 

In  order  to  produce  an  echo,  it  is  requisite  that  the  reflecting  body  should 
be  situated  at  such  a  distance  from  the  source  of  sound,  that  the  interval  be-^ 


198 


WELLS'S   NATURAL   PHILOSOPHY. 


tween  the  perception  of  the  original  and  reflected  sounds  may  be  sufficient  to 
prevent  them  from  being  blended  together. 

"When  the  original  and  reflected  sounds  are  blended  together,  the  effect 
produced  is  called  a  resonance,  and  not  an  echo. 

Thus,  the  walls  of  a  room  of  ordinary  size  do  not  produce  an  echo,  because 
the  reflecting  surface  is  so  near  the  source  of  sound  that  the  echo  is  blended 
with  the  original  sound ;  and  the  two  produce  but  one  impression  on  the  ear. 

Large  halls,  spacious  churches,  etc.,  on  the  contrary,  often  reverberate  or  re- 
peat the  voice  of  a  speaker,  because  the  walls  are  so  far  off  from  the  speaker, 
that  the  echo  does  not  get  back  in  time  to  blend  with  the  original  sound ; 
and  therefore  each  is  heard  separately. 

The  shortest  interval  sufficient  to  render  sounds  distinctly  appreciable  by 
tb*  ear,  is  about  l-9th  of  a  second ;  therefore  when  sounds  follow  at  shorter 
intervals,  they  will  form  a  resonance  instead  of  an  echo ;  so  that  no  reflecting 
surface  will  produce  a  distinct  echo,  unless  its  distance  from  the  spot  where 
the  sound  proceeds  is  at  least  62£  feet ;  as  the  sound  will  in  its  progress  in 
passing  to  and  from  the  reflecting  surface,  at  the  rate  of  1,125  feet  per  sec- 
ond, occupy  l-9th  part  of  a  second,  passing  over  62-^X2  =  125  feet. 

when  is  an  ^®'  Where  separate  surfaces  are  so  situ- 
multi"  atec*  *kat  *key  receive  and  reflect  the  sound 
from  one  to  the  other  in  succession,  multiplied 
echoes  are  heard. 

An  echo  in  a  build- 
ing near  Milan,  Italy, 
^f  repeats  a  loud  sound  30 

times  audibly.  A  river 
bounded  by  perpen- 
dicular walls  of  rock, 
where  the  sound  is  re- 
flected backward  and 
forward  over  the  sur- 
face of  still  water,  is  a 
favorable  situation  for 
the  production  of  re- 
peated echoes.  Fig. 
189  represents  the 
manner  in  which  the 
sounds  rebound,  in  such 
situations,  as  at  1,  2,  3,  4,  from  side  to  side. 

It  is  not  necessary  that  the  surface  producing  an  echo 
tK  of  surface  should  be  either  hard  or  polished.  It  is  often  observed  at 
are  requisite  to  sea  that  an  echo  proceeds  from  the  surface  of  clouds.  An 
echo  at  sea,  however,  or  on  an  extensive  plane,  is  heard  but 
rarely,  there  being  no  surfaces  to  reflect  sound.  To  insure  a 
perfect  echo,  the  reflecting  surface  must  be  tolerably  smooth,  and  of  some 


produce  a  per 
feet  echo? 


REFLECTION   OF   SOUND.  199 

regular  form.  An  irregular  surface  must  break  the  echo  ;  and  if  the  irregu- 
larity be  very  considerable,  there  can  be  no  distinct  or  audible  reflection  at 
all.  For  this  reason  an  echo  is  much  less  perfect  from  the  front  of  a  house 
which  has  windows  and  doors,  than  from  the  plane  end,  or  any  plane  wall  of 
the  same  magnitude, 

HOW  is  sound  431 .  If  the  surface  upon  which  the  sound- 
curvedd  fl6u™  waves  strike  be  concave,  it  may  concentrate 
faces?  sound,  and  reflect  all  that  falls  upon  it  to  a 

point  at  some  distance  from  the  surface,  called  the  focus. 

FlG  19Q  Thus,  in  Fig.  190,  if  the  sound  waves 

proceeding  in  right  lines  from  the  points 
*    d,  e,  /,  ft  A,  strike  upon  the  concave  sur- 

<V  " *    face,  ABC,  they  will  all  be  reflected  to 

v    /    the  focus,  F,  and  there  concentrated  in 

y    such  a  way  as  to  produce  a  most  powerful 

. Jl    effect. 

It  is  upon  this  principle  that  whisper- 
ing galleries  are  constructed,  and  domes  and  vaulted  ceilings  often  exhibit  the 
same  curious  phenomena.  In  these  instances  a  whisper  uttered  at  one  point 
is  reflected  from  the  curved  surface  to  a  focus  at  a  distant  point,  at  which 
situation  it  may  be  distinctly  heard,  while  in  all  other  positions  it  will  be  in- 
audible. 

What         .  All  are  familiar  with  the  resonance  produced  by  placing  a 

the  noise  heard  sea-shell  to  the  ear — an  effect  which  fancy  has  likened  to  the 
in  a  sea-shell  ?  «  roar  of  the  gea  »  Thig  is  cause(j  fcy  tne  hollow  form  of  the 

shell  and  its  polished  surface  enabling  it  to  receive  and  return  the  beatings 
of  all  sounds  that  chance  to  be  trembling  in  the  air  around  the  shell. 

432.  Speaking-tubes  and  speaking-trumpets  depend  on  the  principles  of  the 
reflection  of  sound. 

FIG.  191. 


what  is  a  '       SPEAKING-TRUMPET  (Fig.  191)  is  a 

speaking-Tram-  hollow  tube  so  constructed  that  the  rays  of 
sound  (proceeding  from  the  mouth  when  ap- 
plied to  it),  instead  of  diverging,  and  being  scattered 
through  the  surrounding  atmosphere,  are  reflected  from  the 
sides,  and  conducted  forward  in  straight  lines,  thus  giving 
great  additional  strength  to  the  voice. 


200  WELLS'S  NATURAL  PHILOSOPHY. 

"FiQ.  192.  The  course  of  the  rays  of 

sound  proceeding  from  the 
mouth  through  this  instru- 
ment, may  be  shown  by 
Fig.  1^2.  The  trumpet  be- 
ing directed  to  any  point,  a 
collection  of  parallel  rays  of 
sound  moves  toward  such 
point,  and  they  rea"h  the 
ear  in  much  greater  number  than  would  the  diverging  rays  which  would  pro- 
ceed from  a  speaker  without  such  an  instrument. 

what  is  an  434.  An  Ear-Trumpet  is,  in  form  and  appli- 
Ear-Trumpet?  cation,  the  reverse  of  a  speaking-trumpet,  but 
in  principle  the  same.  The  rays  of  sound  proceeding  from 
a  speaker,  more  or  less  distant,  enter  the  hearing-trumpet 
and  are  reflected  in  such  a  manner  as  to  concentrate  the 
sound  upon  the  opening  of  the  ear. 

FIG  193  ^£'  *^  represents  the  form  of  the  ear-trumpet  gen- 

erally used  by  deaf  persons.  The  aperture  A  is  placed 
within  the  ear,  and  the  sound  which  enters  at  B  is,  by  a 
series  of  reflections  from  the  interior  of  the  instrument, 
concentrated  at  A. 

In  the  same  manner  persons  hold  the  hand  concave 
behind  the  ear,  in  order  to  hear  more  distinctly.  The 
concave  hand  acts,  in  some  respects,  as  an  ear-trumpet,  and  reflects  the  sound 
into  the  ear. 

Most  of  the  stories  in  respect  to  the  so-called  "  haunted  houses"  can  be  all 
satisfactorily  explained  by  reference  to  the  principles  which  govern  the  re- 
flection of  sounds.  Owing  to  a  peculiar  arrangement  of  reflecting  walls  and 
partitions,  sounds  produced  by  ordinary  causes  are  often  heard  in  certain 
localities  at  remote  distances,  in  apparently  the  most  unaccountable  manner. 
Ignorant  persons  become  alarmed,  and  their  imagination  connects  the  phe- 
nomenon with  some  supernatural  cause. 

435.  A  right  understanding  of  the  principles  which  govern  the  reflection 
of  sound  is  often  of  the  utmost  importance  in  the  construction  of  buildings 
intended  for  public  speaking,  as  halls,  churches,  etc. 

Experience  shows  that  the  human  voice  is  capable  of  filling  a  larger  space 
than  was  ever  probably  inclosed  within  the  walls  of  a  single  room. 

The  circumstances  which  seem  necessary  in  order  that  the 
Sic'es  a!-""™"  human  voice  should  be  heard  to  the  greatest  possible  distance, 
essary  to  in-  an(j  with  the  greatest  distinctness,  seem  to  be,  a  perfectly 
Snc'tness^in  tranquil  and  uniformly  dense  atmosphere,  the  absence  of  all 
hearing?  extraneous  sounds,  the  absence  of  echoes  and  reverberations, 

and  the  proper  arrangement  of  the  reflecting  surfaces. 


ORGANS   OF   HEARING  AND   OF  THE  VOICE.  201 

A  pure  atmosphere  in  a  room  for  speaking,  being  favorable 
furl  datemo  *  to  the  speaker's  health  and  strength,  will  give  him  greater 
phereinaroom  pOwer  of  voice  and  more  endurance,  thus  indirectly  improv- 


ing the  hearing  by  strengthening  the  source  of  sound,  and  also 
by  enabling  the  hearer  to  give  his  attention  for  a  longer  period  undisturbed. 

In  constructing  a  room  for  public  speaking,  the  ceiling 
flTpub!      ought  not  to  exceed  30  to  35  feet  in  height. 


he  speaking  be  ^ne  reasOa  of  this  may  be  explained  as  follows: — If  we 
constructed?  "!  .  , 

advance  toward  a  wall  on  a  calm  day,  producing  at  each  step 

What  is  the  SOme  sound,  we  will  find  a  point  at  which  the  echo  ceases 
to  be  distinguishable  from  the  original  sound.  The  distance 
from  the  wall,  or  the  corresponding  interval  of  time,  has  been  called  the  limit 
of  perceptibility.  This  limit  is  about  30  to  35  feet ;  and  if  the  ceiling  of  a 
building  for  speaking  be  arranged  at  this  limit,  the  sound  of  the  voice  and  the 
echo  will  blend  together,  and  thus  strengthen  the  voice  of  the  speaker. 

If  the  ceiling  be  constructed  higher  than  this  limit  of  perceptibility,  or 
higher  than  30  or  35  feet,  the  direct  sound  and  the  echo  will  be  heard  sepa^ 
rately,  and  will  produce  indistinctness. 

Echoes  from  walls  and  ceilings  may,  to  a  certain  extent, 
echoes  in    a-     be  avoided  by  covering  their  surfaces  with  thick  drapery, 
so me ie "extent      wnicn  absorbs  sound,  and  does  not  reflect  it. 
be  avoided  ?  If  the  room  is  not  very  large,  a  curtain  behind  the  speaker 

impedes  rather  than  assists  his  voice. 

436.  In  every  apartment,  owing  to  the  peculiar  arrange- 
nbte  of  an  ment  of  the  reflecting  surfaces,  some  notes  or  tones  can  be 
apartn  heard  with  greater  distinctness  than  others;  or,  in  other 

words,  every  apartment  is  fitted  to  reproduce  a  certain  note,  called  the  key- 
note, better  than  any  other.  If  a  speaker,  therefore,  will  adapt  the  tones  of 
his  voice  to  coincide  with  this  key-note,  which  may  readily  be  determined 
by  a  little  practice,  he  will  be  enabled  to  speak  with  greater  ease  and  distinct- 
ness than  under  any  other  circumstances. 

In  a  large  room  nearly  square,  the  best  place  to  speak  from  is  near  one  cor- 
ner, with  the  voice  directed  diagonally  to  the  opposite  corner.  In  most  cases, 
the  lowest  pitch  of  voice  that  will  reach  across  the  room  will  be  the  most 
audible.  In  all  rooms  of  ordinary  form,  it  is  better  to  speak  along  the  length 
of  a  room  than  across  it.  It  is  better,  generally,  to  speak  from  pretty  near  a 
wall  or  pillar,  than  far  away  from  it. 

SECTION    III. 

ORGANS  OP  HEARING  AND  OT  THE  TOICE. 

437.  The  Ear  consists,  in  the  first  instance,  of  a  funnel- 
construction  of  shaped  mouth,  placed  upon  the  external  surface  of  the  head, 
the  human  ear.  In  many.  animais  tnjs  [s  movable,  so  that  they  can  direct  it 
to  the  place  from  whence  the  sound  comes.  It  is  represented  at  a,  Fig.  194. 

9* 


202 


WELLS'S   NATUKAL  PHILOSOPHY. 


FIG.  194.  Proceeding  inward  from  this  external  por- 

tion of  the  ear,  is  a  tube,  something  more  than 
an  inch  long,  terminating  in  an  oval-shaped 
opening,  &,  across  which  is  stretched  an  elas- 
tic membrane,  like  the  parchment  on  the  head 
of  a  drum.  This  oval-shaped  opening  has  re- 
ceived the  name  of  the  tympanum,  or  drum  of 
the  ear,  and  the  membrane  stretched  across  it 
is  called  the  "membrane  of  the  tympanum,  or 
drum  of  the  ear." 

The  sound  concentrated  at  the  bottom  of  the  ear-tube  falls  upon  the  mem- 
brane of  the  drum,  and  causes  it  to  vibrate.  That  its  motion  may  be  free, 
the  air  contained  within  and  behind  the  drum  has  free  communication  with 
the  external  air  by  an  open  passage.  /,  called  the  eustachian  tube,  leading  to 
the  back  of  the  mouth.  A  degree  of  deafness  ensues  when  this  tube  is  ob- 
structed, as  in  a  cold;  and  a  crack,  or  sudden  noise,  with  immediate  return 
of  natural  hearing,  is  generally  experienced  when,  in  the  effort  of  sneezing  or 
otherwise,  the  obstruction  is  removed.  - 

The  vibrations  of  the  membrane  of  the  drum  are  conveyed  further  inward, 
through  the  cavity  of  the  drum,  by  a  chain  of  four  bones  (not  represented  in 
the  figure  on  account  of  their  minuteness),  reaching  from  the  center  of  the 
membrane  to  the  commencement  of  an  inner  compartment  which  contains  the 
nerves  of  hearing.  This .  compartment,  from  its  curious  and  most  intricate 
structure  is  called  the  Labyrinth.  Pig.  194,  c  e  d. 

The  Labyrinth  is  the  true  ear,  all  the 
other  portions  being  merely  accessories  by 
which  the  sonorous  undulations  are  propa- 
gated to  the  nerves  of  hearing  contained 
in  the  labyrinth,  which  is  excavated  in  the 
hardest  mass  of  bone  found  in  the  whole 
body.  Fig.  195  represents  the  labyrinth 
on  an  enlarged  scale,  and  partially  open. 

The  labyrinth  is  filled  with  a  liquid  sub- 
stance, through  which  the  nerves  of  hearing 
are  distributed.  "When  the  membrane  of 
the  drum  of  the  ear  is  made  to  vibrate  by  the  undulations  of  sound  striking 
against  it,  the  vibrations  are  communicated  to  the  little  chain  of  bones, 
Which,  in  turn,  striking  against  a  membrane  which  covers  the  external 
opening  of  the  labyrinth,  compresses  the  liquid  contained  in  it.  This  ac- 
tion, by  the  law  of  fluid  pressure,  is  communicated  to  the  whole  interior  of 
the  labyrinth,  and  consequently  to  all  portions  of  the  auditory  nerve  dis- 
tributed throughout  it:  the  nerve  thus  acted  upon  conveys  an  impression  to 
the  brain. 

The  several  parts  of  the  labyrinth  consist  of  what  is  called  the  vestibule, 
tf,  Fig.  194,  three  semicircular  canals,  c,  imbedded  in  the  hard  bone,  and  a 
winding  cavity,  called  the  cochlea,  d,  like  that  of  a  snail-shell,  in  which  fibres, 


FIG.  195. 


ORGANS   OF   HEARING   AND   OF   THE   VOICE.  203 

.stretched  across  like  harp-strings,  constitute  the  lyra.  The  separate  uses  of 
these  various  parts  are  not  yet  fully  known.  The  membrane  of  the  tym- 
panum may  be  pierced,  and  the  chain  of  bones  may  be  broken,  without  en- 
tiro  loss  of  hearing. 

438.  In  the  hearing  apparatus  of  the  lower  orders  of 
Pof  anima!s>  a11  tne  Parts  belonging  to  the  human  ear  do  not 
the  hearing  ap-  exist.  In  fishes,  the  ear  consists  only  of  the  labyrinth;  and 
in  lower  animals  tlie  ear  is  simply  a  little  membranous 
cavity  filled  with  fluid  in  which  the  fibres  of  the  nerves  of 
hearing  float. 

439.  All  persons  can  not  hear  sounds  alike. 
?onns  aU  E     In  different  individuals   the  sensibility  of  the 
sound  alike?       auditory  nerves  varies  greatly. 

440.  The  whole  range  of  human  hearing, 

What    is    the  •  £•*  11-    i 

range  of  hn-  from  the  lowest  note  of  the  organ  to  the  high- 
man  hearing?  "  . 

est  known  cry  of  insects,  as  of  the  cricket,  in- 
cludes ahout  nine  octaves. 

441.  In  the  human  system,  the  parts  con- 

Wh.it  are  the  i    •         i  e*  i 

organs       of    cerned  in  the  production  01  speech  and  music. 

voice?  i  •      1       •  i          i  i 

are  three :    the  wind-pipe,    the   larynx,    and 
the  glottis.  '  "4£'.? 

wimt  is  the        442.  The  Wind-pipe  is  a  tube   extending 
wind-pipe?      from  Qne  extremity  of  the  throat  to  the  other, 

which  terminates  in  the  lungs,  through  which  the   air 

passes  to  and  from  these  organs  of  respiration. 

what  is  the        443.  The  Larynx,  which  is   essentially  the 

Larynx?  organ  of  speech,  is  an  enlargement  of  the  up- 
per part  of  the  wind-pipe.  The  Larynx  terminates  in 
two  lateral  membranes  which  approach  near  to  each  other, 
having  a  little  narrow  opening  between  them  called  the 
glottis.  The  edges  of  these  membranes  form  what  is 
called  the  vocal  chords. 
HOW  is  voice  444.  In  order  to  produce  sound,  the  air  ex- 

produced?  pired  from  the  lungs  passes  through  the  wind- 
pipe and  out  at  the  larynx,  through  the  opening  between 
the  membranes,  the  glottis  :  the  vibration  of  the  edges  of 
these  membranes,  caused  by  the  passage  of  air,  produces 
sound. 


204  WELLS'S   NATURAL   PHILOSOPHY. 

By  the  action  of  muscles  wo  can  vary  the  tension  of  these 
How  can  the  J 

tones    of   the      membranes,  and  make  the  opening  between  them  large  or 

dered  graved      smau>  and  tnus  render  the  tones  of  the  voice  grave  or  acute.* 

acute?  445.    The .  loudness    of  the  voice   depends 

SeTttil^oud-     mainly  upon  the  force  with  which  the  air  is 
SepSeSd"lev°ice    exPelled  fr°m  tne  lungs. 

The  force  which  a  healthy  chest  can  exert  in  blowing  is 
about  one  pound  per  inch  of  its  surface ;  that  is  to  say,  the  chest  ca<n  con- 
dense its  contained  air  with  that  force,  and  can  blow  through  a  tube  the 
mouth  of  which  is  ten  feet  under  the  surface  of  water. 

446.  Iii  coughing,  the  top  of  the  windpipe, 
°f  or  the  glottis>  is  closed  for  an  instant,  during 
which  the  chest  is  compressing  and  condensing 
its  contained  air  ;  and  on  the  glottis  being  opened,  a 
slight  explosion,  as  it  were,  of  the  compressed  air  takes 
place,  and  blows  out  any  irritating  matter  that  may  be 
in  the  air-passages. 

D  .  447.  Sound,  to  some  extent,  appears  to  always  accompany 

generally    ac-      the  liberation  of  compressed  air.     An  example  of  this  is  seen 
HbenTuon   *^>f     *n  ^e  r°Por*  wu*ch  a  pop-gun  makes  when  a  paper-bullet 
compressed  air  ?    is  discharged  from  it.     The  a^r  confined  between  the  paper 
bullet   and   the   discharging-rod   is   suddenly   liberated,  and 
strikes  against  the  surrounding  air,  thus  causing  a  report  in  the  same  man- 
ner as  when  two  solids  come  into  collision.     In  like  manner  an  inflated  blad- 
der, when  burst  open  with  force,  produces  a  sound  like  the  report  of  a  pistol. 
.  448.  The  sound  of  falling  water  appears  in  a  great  measure 

sound  of  falling     to  be  owing  to  the  formation  and  bursting  of  bubbles.     When 
water  due?          jue  distance  which  water  falls  is  so  limited  that  the  end  of 

*  The  power  which  the  will  possesses  of  determining  with  the  most  perfect  precision 
the  exact  degree  of  tension  which  these  membranes  of  the  glottis,  or  vocal  chords  shall 
receive,  is  extremely  remarkable.  Their  average  length  in  man  is  estimated  at  73-100ths 
of  an  inch  in  a  state  of  repose,  while  in  the  state  of  greatest  tension  it  is  about  93-100th3 
of  an  inch.  The  average  length  of  the  membranes  in  the  female  is  somewhat  less.  Each 
interval,  or  variation  of  tone  which  the  human  voice  is  capable  of  producing  is  occasioned 
by  a  different  degree  of  tension  of  these  membranes  :  and  as  the  least  estimated  number 
of  variations  belonging  to  the  voice  is  240,  there  must  be  240  different  states  of  tension 
of  the  vocal  chords,  or  membranes,  every  one  of  which  can  be  at  once  determined  by  the 
will.  Their  whole  variation  in  length  in  man  being  not  more  than  one  fifth  of  an  inch, 
the  variation  required  to  pass  from  one  interval  of  tone  to  another  will  not  be  more  than 
l-1200th  of  an  inch. 

It  is  on  account  of  the  greater  length  of  the  vocal  chords,  or  membranes  of  the  glottis, 
that  the  pitch  of  the  voice  is  much  lower  in  man  than  in  woman  :  but  the  difference  does 
not  arise  until  the  end  of  the  period  of  childhood,,  the  size  of  the  larynx  in  both  sexes  being 
about  the  same  up  to  the  age  of  14  or  15  years,  but  then  changing  rapidly  in  the  male 
sex,  and  remaining  nearly  stationary  in  the  female.  Hence  it  is  that  boys,  as  well  as 
girls  and  women,  sing  treble ;  while  men  sing  tenor,  which  is  about  an  octave  lower  than 
treble,  or  bass  which  in  lower  still.— Dr.  Carpenter. 


HEAT.  -205 

the  stream  docs  not  become  broken  into  bubbles  and  drops,  neither  sound  or 
air-bubbles  will  be  produced  ;  but  as  soon  as  the  distance  becomes  increased 
to  a  sufficient  extent  to  break  the  end  of  the  column  into  drops,  both  air-bub- 
bles and  sounds  will  be  produced. 

whatissneez-        449.  Sneezing  is  a  phenomenon  resembling 
*"*'  cough  ;  only  the  chest  empties  itself  at  one 

effort,  and  chiefly  through  the  nose,  instead  of  through 
the  mouth,  as  in  coughing. 

what  is  laugh-  450.  Laughing  consists  of  quickly-repeated 
mg?  expulsions  of  air  from  the  chest,  the  glottis 
being  at  the  time  in  a  condition  to  produce  voice  ;  but 
there  is  not  between  the  expirations,  as  in  coughing,  a 
complete  closure  of  the  glottis. 

whatis crying?       451.    Crying  differs  from  laughing  almost 
solely  in  the  circumstance  of  the  intervals  be- 
tween the  gusts  or  expirations  of  air  from  the  lungs  being 
longer.     Children  laugh  and  cry  in  the  same  breath. 

Insects  generally  excite  sonorous  vibrations  by  the  fluttering  of  their  wings, 
or  other  membraneous  parts  of  their  structure. 

PRACTICAL    QUESTIONS   IN   ACOUSTICS. 

1.  The  flash  of  a  cannon  was  seen,  and  ten  seconds  aftenrard  the  report  was  heard : 
how  far  off  was  the  cannon  ? 

2.  At  what  distance  was  a  flash  of  lightning,  when  the  flash  was  seen  seven  seconds 
before  the  thunder  was  heard  ? 

3.  How  long  after  a  sudden  shout  will  an  echo  he  returned  from  a  high  wall  1,120  feet 
distant  ? 

4.  A  stone  being  dropped  into  the  mouth  of  ft  mine,  was  heard  to  strike  the  bottom 
in  two  seconds  ;  how  deep  was  the  mine  ? 

5.  A  certain  musical  string  vibrates  100  times  in  a  second :  how  many  times  must  it 
vibrate  in  a  second  to  produce  the  octave  ? 


CHAPTER    III. 

HEAT. 

heat?  '  -^-EAT  *s  a  physical  agent,  known  only 

by  its  effects  upon  matter.  In  ordinary  lan- 
guage we  use  the  term  heat  to  express  the  sensation  of 
warmth. 


206  WELLS'S  NATURAL  PHILOSOPHY. 

mat  is  caioricr      453.  Calorie  is  the  general  name  given  to 
the  physical  agent  which  produces  the  sensa- 
tion of  warmth,  and  the  various  effects  of  heat  observed 
in  matter. 

HOW  is  heat  454.  The  quantity  of  heat  observed  in  dif- 
ferent substances  is  measured,  and  its  effects 
on  matter  estimated,  only  by  the  change  in  bulk,  or  ap- 
pearance, which  different  bodies  assume,  according  as  heat 
is  added  or  subtracted. 

what  is  tem-        455.  The  degree  of  heat  by  which  a  body 
perature?        -g  affected,  or  the  sensible  heat  a  body  con- 
tains, is  called  its  TEMPERATURE. 

What  is  coid?        456.  Cold  is  a  relative  term  expressing  only 
the  absence  of  heat  in  a  degree  ;  not  its  total 
absence,  for  heat  exists  always  in  all  bodies, 
what   distin-        457.  Heat  possesses  a  distinguishing  char- 
IcteriSfcCdoIs     acteristic  of  passing  through  and  existing  in 

heat  possess?         ^  fo^fe  of  matter  at  ft]J  tjmes>        gQ  far  ag  W0 

know,  heat  is  everywhere  present,  and  every  body  that 
exists  contains  it  without  known  limits. 

Ice  contains  heat  in  large  quantities.  Sir  Humphrey  Davy,  by  friction,  ex- 
tracted heat  from  two  pieces  of  ice,  and  quickly  melted  them,  in  a  room  cooled 
below  the  freezing-point,  by  rubbing  them  against  each  other. 

in  what  man-  458.  The  tendency  of  heat  is  to  diffuse,  or 
diffuse,68  oreat  spread  itself  among  all  neighboring  substances, 
spread  itself ?  until  all  have  acquired  the  same,  or  a  uniform 
temperature. 

A  piece  of  iron  thrust  into  burning  coals  becomes  hot  among  them,  because 
the  heat  passes  from  the  coals  into  the  iron,  until  the  metal  has  acquired  an 
equal  temperature. 

when  do  we  459.  When  the  hand  touches  a  body  having 
caiiabodyhot?  ft  hig^j  temperature  than  itself,  we  call  it 
hot,  because  on  account  of  the  law  that  heat  diffuses  itself 
among  neighboring  bodies  until  all  have  acquired  the 
same  temperature,  heat  passes  from  the  body  of  higher 
temperature  to  the  hand,  and  causes  a  peculiar  sensation, 
which  we  call  warmth. 
460.  When  we  touch  a  body  having  a  temperature 


HEAT.     ,  207 

When  do  we  lower  than  that  of  the  hand,  heat,  in  accord- 
can  a  body  coid?  ance  i^th  |[ie  same  law,  passes  out  from  the 
hand  to  the  body  touched,  and  occasions  the  sensation 
which  we  call  cold.* 

461.  Sensations  of  heat  and  cold  are,  therefore,  merely 
degrees  of  temperature,  contrasted  by  name  in  reference 
to  the  peculiar  temperature  of  the  individual  speaking  of 
them. 

Under     what  ^  bod^  ma^  feel  hot  &lld  C°ld  tO  tte   Same   Person   at  the 

eircumstances       same  time,  since  the  sensation  of  heat  is  produced  by  a  body 

hoTandSTo1  C°ldei>  thaEL  the  hand'  Provided  &  be  less  cold  than  the  b°d7 
the  same  per-  touched  immediately  before ;  and  the  sensation  of  cold  is 
time*?  Sam8  produced  under  the  opposite  circumstances,  of  touching  a 
comparatively  warm  body,  but  which  is  less  warm  than  some 
other  body  touched  previously.  Thus,  if  a  person  transfer  one  hand  to  com- 
mon spring  water  immediately  after  touching  ice,  to  that  hand  the  water 
would  feel  very  warm  ;  while  the  other  hand  transferred  from  warm  water 
to  the  spring  water,  would  feel  a  sensation  of  cold. 

Has   heat  462.  Heat  is  imponderable,  or  does  not  pos- 

•  weight?          gegg  any  perceptible  weight. 

If  we  balance  a  quantity  of  ice  in  a  delicate  scale,  and  then  leave  it  to 
melt,  the  equilibrium  will  not  be  in  the  slightest  degree  disturbed.  If  we 
substitute  for  the  ice  boiling  water  or  red-hot  iron,  and  leave  this  to  cool, 
there  will  be  no  difference  in  the  result.  Count  Kumford,  having  suspended 
a  bottle  containing  water,  and  another  containing  alcohol  to  the  arms  of  a 
balance  and  adjusted  them  so  as  to  be  exactly  in  equilibrium  found  that  the 
balance  remained  undisturbed  when  the  water  was  completely  frozen,  though 
the  heat  the  water  had  lost  must  have  been  more  than  sufficient  to  have  made 
an  equal  weight  of  gold  red  hot. 

what  do  we  463.  The  nature,  or  cause  of  heat  is  not 
StawrfheX8?  clearly  understood.  Two  explanations,  or 
theories  have  been  proposed  to  account  for  the 
various  phenomena  of  heat,  which  are  known  as  the  me- 
chanical and  vibratory  theories. 

Explain  the  me-        464.  The  mechanical  theory  supposes  heat 
chamcai  theory.   to  ^  ^  extrenieiy  gubtile  fluid,  or  etherial 

*  There  can  not  be  a  more  fallacious  means  of  estimating  heat  than  by  the  tonch.  Thus, 
in  the  ordinary  s^ate  of  an  apartment,  at  any  season  of  the  year,  the  objects  which  are  in 
it  have  all  the  same  temperature ;  and  yet  to  the  touch  they  will  feel  warm  and  cold  in 
different  degrees.  The  metallic  objects  will  be  the  coldest;  stone  and  marble  less  so; 
wood  still  less;  and  carpeting  and  woolen  objects  will  feel  warm.  Now  all  these  objects 
are  at  exactly  the  same  temperature,  as  ascertained  by  the  thermometer. 


208  WELLS'S   NATURAL   PHILOSOPHY. 

kind  of  matter  pervading  all  space,  and  entering  into 
combination  in  various  proportions  and  quantities,  with, 
all  bodies,  and  producing  by  this  combination  all  the  va- 
rious effects  noticed. 

Explain  the  ri-  465.  The  vibratory  theory,  on  the  contrary, 
bratory  theory.  SUppOses  heat  to  be  merely  the  effect  of  a  spe- 
cies of  motion,  like  a  vibration  or  undulation,  produced 
either  in  the  constituent  particles  of  bodies,  or  in  a  subtle, 
imponderable  fluid  which  pervades  them. 

"When  one  end  of  a  bar  of  iron  is  thrust  into  the  fire  and  heated,  the  other 
end  soon  becomes  hot  also.  According  to  the  mechanical  theory,  a  subtile 
fluid  coming  out  of  the  fire  enters  into  the  iron,  and  passes  from  particle  to 
particle  until  it  has  spread  through  the  whole.  "When  the  hand  is  applied  to 
the  bar  it  passes  into  it  also,  and  occasions  the  sensation  of  warmth.  Ac- 
cording to  the  vibratory  theory,  the  heat  of  the  fire  communicates  to  the  par- 
ticles of  the  iron  themselves,  or  to  a  subtile  fluid  pervading  them,  certain  vi- 
bratory motions,  which  motions  are  gradually  transmitted  in  every  direction, 
and  produce  the  sensation  of  heat,  in  the  same  way  that  the  undulations  or 
vibrations  of  air,  produce  the  sensation  of  sound. 

There  seems  to  be  but  little  doubt  at  the  present  time  among 
two     theories      scientific  men,  that  the  theory  which  ascribes  the  phenome- 
'*"      na  °^  ^ea*  *°  a  ser*es  °^  vibrations,  or  undulations,  either  in, 

matter,  or  a  fluid  pervading  it,  is  substantially  correct.  At 
the  same  time  it  is  not  wholly  satisfactory,  and  neither  theory  will  perfectly 
explain  all  the  facts  in  relation  to  heat  with  which  we  are  acquainted. 
For  the  purpose  of  describing  and  explaining  the  phenomena  and  effects  of 
heat,  it  is  convenient,  in  many  cases,  to  retain  the  idea  that  heat  is  a  substance. 
The  fact  that  nature  nowhere  presents  us,  neither  has  art 
dencosinfavor  ever  succeeded  m  showing  us,  heat  alone  in  a  separate  state, 
of  the  respect-  is  a  strong  ground  for  believing  that  heat  has  no  separate 
heat*60"6*  material  existence.  Heat,  moreover,  can  be  produced  without 

limit  by  friction,  and  intense  heat  is  also  produced  by  the  ex- 
plosion of  gunpowder.  On  the  contrary,  as  arguments  in  favor  of  the  material 
existence  of  heat,  we  have  the  fact,  that  heat  can  be  communicated  very 
readily  through  a  vacuum ;  that  it  becomes  instantly  sensible  on  the  condens- 
ation of  any  material  mass,  as  if  it  were  squeezed  out  of  it :  as  when,  on  re- 
ducing the  bulk  of  a  piece  of  metal  by  hammering,  we  render  it  very  hot  (the 
greatest  amount  of  heat  being  emitted  with  the  blows  that  most  change  its 
bulk) ;  and,  finally,  that  the  laws  of  the  spreading  of  heat  do  not  resemble 
those  of  the  spreading  of  sound,  or  of  any  other  motion  known  to  us. 

466.  The  relation  between  heat  and  light  is  a  very  intimate 
IT*  thereatj£!  one-  Heat  exists  without  nSht>  but  all  the  ordinary  sources 
tween  heat  and  of  light  are  also  sources  of  heat;  and  by  whatever  artificial 

means  natural  light  is  condensed,  so  as  to  increase  ita  eplen- 


SOURCES   OF   HEAT.  209 

dor,  the  heat  which  it  produces  is  also,  at  the  same  time,  rendered  more 
intense. 

467.  When  a  body,  naturally  incapable  of 
bodyincandes-     emitting  light,  is  heated  to  a  sufficient  extent 
to  become  luminous,  it  is  said  to  bo  incandes- 
cent, or  ignited. 

what  is  flame        468.    Flame    is   ignited    gas   issuing    from 
and  fire?        a  kurnmg  body.     Fire  is  the  appearance  of 
heat  and  light  in  conjunction,  produced  by  the  combus- 
tion of  inflammable  substances. 

The  ancient  philosophers  used  the  term  fire  as  a  characteristic  of  the  nature 
of  heat,  and  regarded  it  as  one  of  the  four  elements  of  nature  ;  air,  earth,  and 
water  being  the  other  three. 

Heat  and  the  attraction  of  cohesion  act  constantly  in  opposition  to  each 
other  ;  hence,  the  more  a  body  is  heated,  the  less  will  be  the  attractive  force 
between  the  particles  of  which  it  is  composed. 

."•  i 

SECTION    I. 

SOURCES     OP     HEAT. 


Tufci  aie  the  469.  *^X  grea*  sources  of  heat  are  recognized. 
sources  of  heat?  They  are  —  1.  The  sun  ;  2.  The  interior  of  the 
earth  ;  3.  Electricity  ;  4.  Mechanical  action  ;  5.  Chemical 
action  ;  6.  Vital  action. 

what  is  the        470.  The  greatest  natural  source  of  heat  is 
Srsourco^of    the  sun,  as  it  is  also  the  greatest  natural  source 
of  light. 

Although  the  quantity  of  heat  sent  forth  from  the  sun  is  immense,  its  rays, 
falling  naturally,  are  never  hot  enough,  even  in  the  torrid  zone,  to  kindle 
_,  combustible  substances.     By  means,  however,  of  a 

burning-glass,  the  heat  of  the  sun's  rays  can  be  con- 
centrated, or  bent  toward  one  point,  called  a  focus, 
in  sufficient  quantity  to  set  fire  to  substances  sub- 
mitted to  their  action. 

Fig.  196  represents  the  manner  in  which  a  burning- 
glass  concentrates  or  bends  down  the'  rays  of  heat 
until  they  meet  in  a  focus. 
Two  opinions,  or  theories,  have  been  entertained  in  order  to  account  for  the 
production  of  heat  and  light  by  the  sun  ;    one  supposes  that  the  sun  is  an 
intensely-heated  mass,  which  throws  off  its  light  and  heat  like  an  intensely- 
heated  mass  of  iron  :  the  other,  based  on  the  ground  that  heat  is  occasioned 


210  WELLES  NATURAL  PHILOSOPHY. 

by  the  vibrations  of  an  ethereal  fluid  occupying  all  space,  supposes  that  tho 
sun  may  produce  the  phenomena  of  light  and  heat  without  waste  of  its  tem- 
perature or  substance,  as  a  bell  may  constantly  produce  tho  phenomena  of 
sound. 

Whatever  may  be  the  true  theory,  a  series  of  experiments,  made  some  years 
since  by  Arago,  the  eminent  French  astronomer,  seem  to  prove  that  tho  tem- 
perature at  the  surface  of  the  sun  is  much  more  elevated  than  any  artificial 
heat  we  are  able  to  produce.  The  experimental  reasons  which  lead  to  this 
opinion  are  as  follows: — 

There  are  two  states  in  which  light  is  capable  of  existing — the  ordinary 
state,  and  the  state  of  polarization.*  It  has  been  proved  that  all  bodies,  in  a 
solid  or  liquid  state,  which  are  rendered  incandescent  by  heat,  emit  a  polar- 
ized light,  while  bodies  that  are  gaseous,  when  rendered  incandescent,  inva- 
riably emit  light  in  its  ordinary  state.  Thus  the  physical  condition  of  a  body 
may  be  distinguished  when  it  is  incandescent  by  examining  the  light  which 
it  affords.  On  applying  the  test  to  the  direct  light  of  the  sun,  it  was  found  to 
be  in  the  unpolarized  or  ordinary  condition  of  light.  Hence  it  has  been  in- 
ferred by  Arago  that  the  matter  from  which  this  light  proceeds  must  be  in 
the  gaseous  state,  or,  in  other  words,  in  a  state  of  flame.  From  other  experi- 
ments and  observations,  Arago  was  led  to  the  conclusion  that  the  sun  was  a 
solid,  opaque,  non-luminous  body,  invested  with  an  ocean  of  flame. 

471.  .Owing  to  the  position  of  the  earth's 

Whyisthercl-  &.  /.  ,  -,    /> 

ative  heat  of    axis,  the  relative  amount  of  heat  received  from 

the  sun  always  7  .  :f3' 7  .«  /. 

greater  in  some    the  sun  is  always  greater  in  some  portions  of 

portions  of  the         ,  *'•••'  ,  .  ,  />      •• 

earth  than  at  the  earth  than  at  others,  since  the  rays  of  the 
sun  always  fall  more  directly  upon  the  central 
portions  of  the  earth  than  they  do  at  the  poles,  or  extremi- 
ties ;  and  the  greatest  amount  of  heat  is  experienced  from 
the  rays  of  the  sun  when  they  fall  most  perpendicularly. 

\Vh      is     the  4'?2'  The  Il6at   °f  tlie  SUn  is  £reatest  at  noon>  because  for 

heat7of  the  sun     the  day  the  sun  has  reached  the  highest  point  in  the  heavens, 

noon?St       at     and  its  rays  fal1  more  perpendicularly  than  at  any  other 
time. 

What  occasions        F0r  a  like  reason  we  experience  the  extremes  of  tempera- 

the     difference  ...         .  ,     , 

in  temperature    ture,  distinguished  as  summer  and  winter.     In  summer  the 

"**  Position  of  the  RUIi  in  relation  to  the  earth  is  such,  that  al- 
though more  remote  from  the  earth  than  in  winter,  its  rays 
fall  more  perpendicularly  than  at  any  other  season,  and  impart  the  greatest 
amount  of  heat ;  while  in  winter  the  position  of  the  sun  -is  such  that  its  rays 
fall  more  obliquely  than  at  any  other  time,  and  impart  the  smallest  amount  of 
heat.  The  sun,  moreover,  is  longer  above  the  horizon  in  summer  than  in 
winter,  which  also  produces  a  corresponding  effect. 

The  reason  why  a  difference  in  the  inclination  of  the  sun's  rays  falling  upon 
'*  For  explanation  of  the  term  polarization,  see  chapter  on  Light. 


SOURCES   OF   HEAT.  211 

the  surface  of  the  earth  occasions  a  difference  in  their  heating  effect  is,  that 
the  more  the  rays  are  inclined,  the  more  they  are  diffused,  or,  in  other  words, 
the  larger  the  space  they  cover.  This  may  be  rendered  apparent  by  reference 
to  Fig.  197. 

FIQ  197  ket  us  8UPPose  A  B  C  D  to  represent 

a  portion  of  the  sun's  rays,  and  C  D  a 
portion  of  the  earth's  surface  upon  which 
the  rays  fall  perpendicularly,  and  C  E 
portions  of  the  surface  upon  which  they 
fall  obliquely.  The  same  number  of 
rays  will  strike  upon  the  surfaces  C  D 
and  C  E,  but  the  surface  C  E  being 
greater  than  C  D,  the  rays  will  necessarily  fall  more  densely  upon  the  latter ; 
and  as  the  heating  power  must  be  in  proportion  to  the  density  of  the  rays,  it 
is  obvious  that  C  D  will  be  heated  more  than  C  E,  in  just  the  same  propor- 
tion as  the  surface  C  E  is  more  extended.  But  if  we  would  compare  two 
surfaces  upon  neither  of  which  the  sun's  rays  fall  perpendicularly,  let  us  take 
C  E  and  C  F.  They  fall  on  C  E  with  more  obliquity  than  on  C  F;  but  C  E 
is  evidently  greater  than  C  F,  and  therefore  the  rays  being  diffused  over  a 
larger  surface  are  less  dense,  and  therefore  less  effective  in  heating. 

what  is  ttu»^  473.  The  greatest  natural  temperature  ever 
SuetJ^S  authentically  recorded  was  at  Bagdad,  in  1819, 
ever  observed?  wnen  tne  thermometer  (Fahrenheit's)  rose  to 
120°  in  the  shade.  On  the  west  coast  of  Africa  the  ther- 
mometer has  been  observed  as  high  as  108°  F.  in  the 
shade.  Burckhardt  in  Egypt,  and  Humboldt  in  South 
America,  observed  it  at  117°  F.  in  the  shade. 

474.  About  70°  below  the  zero  of  Fahrenheit's 

What    is    the         ,  .  . 

lowest  tempe-    thermometer  is  the  lowest  atmospheric  temper- 

rature  observed?  .  1-1,11.  . 

ature  ever  experienced  by  the  Arctic  navigators. 
Towhatextent  475.  The  greatest  artificial  cold,  ever  pro- 
eoTd  b£fptl-  duced  was  220°  F.  below  zero. 

.  duced  ?  This  temperature  was  obtained  some  years  since  by  M. 

Natterer,  a  German  chemist.  Professor  Faraday  also  produced  a  cold 
equal  to  166°  F.  below  zero.  At  neither  of  these  temperatures  were  pure 
alcohol  or  ether  frozen. 

The  temperature  of  the  space  above  tho  earth's  atmosphere  has  been  esti- 
mated at  58°  below  zero,  Fahrenheit's  thermometer. 

TO  what  depth  476.  The  depth  to  which  the  influence  of 
dowtlthee!£2  tne  heat  of  tne  sun  extends  into  the  earth  va- 
e*.  ries  from  50  to  100  feet  ;  never,  however,  ex- 
ceeding  the  latter  distance. 


212  WELLS'S  NATURAL  PHILOSOPHY. 

How    do    we          Independently  of  the  sun,  however,  the  earth  is  a  source  of 
know  that  the      heat.     The  proof  of  this  is  to  be  found  in  the  fact,  that  as  we 
ofrheutS?a80UrCe    descend  into  the  earth,  and  pass  beyond  the  limits  of  the  influ- 
ence of  the  solar  heat,  the  temperature  constantly  rises. 

At  what  rate        477.  The  increase  of  temperature  observed  as 

pemtureofethe    we  descend  into  the  earth,  is  about  one  degree 

of  the  therrnometerfor  every  fifty  feet  of  descent. 

Supposing  the  temperature  to  increase  according  to  this  ratio,  at  the  depth 
of  two  miles  water  would  be  converted  into  steam ;  at  four  miles,  tin  would 
be  melted ;  at  five  miles,  lead ;  ana  at  thirty  miles,  almost  every  earthy  sub- 
stance would  be  reduced  to  a  fluid  state. 

The  internal  heat  of  the  earth  does  not  appear  to  have  any  sensible  effect 
upon  the  temperature  at  the  surface,  being  estimated  at  less  than  l-30th  of 
a  degree.  The  reason  why  such  an  amount  of  heat  as  is  supposed  to  exist  in 
the  interior  of  the  earth  does  not  more  sensibly  affect  the  surface  is  because 
the  materials  of  which  the  exterior  strata  or  crust  of  the  earth  is  composed, 
do  not  conduct  it  to  the  surface  from  the  interior. 

under  what  478.  When  electricity  passes  from  one  sub- 

isdeSriS'ty6*  stance  to  another,  the  medium  which  serves  to 
source  of  heat?  con(juct  it  is  very  frequently  heated ;  but  in 
what  manner  the  heat  is  produced  we  have  no  positive  in- 
formation. 

The  greatest  known  heat  with  which  we  are  acquainted,  is  thus  produced 
by  the  agency  of  the  electric  or  galvanic  current.  All  known  substances  can 
be  melted  or  volatilized  by  it. 

Heat  so  developed  has  not  been  employed  for  practical  or  economical  pur- 
poses to  any  great  extent ;  but  for  philosophical  experiments  and  investiga- 
tions it  has  been  made  quite  useful. 

HOW  is  chem-  479.  Many  bodies,  when  their  original  con- 
BouVceofheat1?  stitution  is  altered,  either  by  the  abstraction 
of  some  of  their  component  parts,  or  by  the 
addition  of  other  substances  not  before  in  combination 
with  them,  evolve  heat  while  the  change  is  taking  place. 

In  such  cases,  the  heat  is  said  to  be  due  to  chemical  ac- 
tion. 

what  is  chem-  480.  We  apply  the  term  chemical  action  to 
icai action?  those  operations,  whatever  they  may  be,  by 
which  the  form,  solidity,  color,  taste,  smell,  and  action  of 
substances  become  changed  ;  so  that  new  bodies,  with 
quite  different  properties,  are  formed  from  the  old. 

A  familiar  illustration  of  the  manner  in  which  heat  is  evolved  by  chemical 


SOURCES   OF    HEAT.  213 

action  is  to  be  found  in  the  experiment  of  pouring  cold  water  upon  quick- 
lime. The  water  and  the  lime  combine  together,  and  in  so  doing  liberate  a 
great  amount  of  heat,  sufficient  to  set  fire  to  combustible  substances. 

now  is  heat  481.  Heat  is  always  evolved  when  a  fluid  is 
cfangedof  form  transformed  into  a  solid,  and  is  always  ab- 
ia matter?  soi'bed  when  a  solid  is  made  to  assume  a  fluid 
condition.  As  water  is  changed  from  its  liquid  form  when 
it  is  taken  up  by  quicklime,  therefore  heat  is  given  off. 

The  heat  produced  by  the  various  forms  of  combustion,  is  the  result  of  chem- 
ical action. 

in  what  two  482.  Heat  exists  in  two  very  different  con- 
conditions  does  ditions,  as  FREE,  or  SENSIBLE  HEAT,  and  as 

heat  exist?  ' 

LATENT  HEAT.* 

what  is  sensi-        483.  When  the  heat  retained  or  lost  by  a 
we  heat?          body  is  attended  with  a  sense  of  increased  or 
diminished  warmth,  it  is  called  sensible  heat. 
what,  is  latent        484.  When  the  heat  retained  or  lost  by  a 

body  is  not  perceptible  to  our  sense,  it  is  called 
latent  heat, 

Every  substance  contains  more  or  less  of  latent  heat.     Al- 
How    do    we  J 

know  heat  to      though  our  senses  give  us  no  direct  information  of  its  pres- 

Sr^V^n°not  ence»  we  may'  by  a  variety  of  experiments,  prove  that  it  ex- 
perceiye  it?  ists.  Thus,  the  temperature  of  ice  is  32°  by  the  thermometer, 
but  if  ice  be  melted  over  a  fire  and  converted  into  water,  the 
water  will  be  no  hotter  than  the  ice  was  before,  although  in  the  operation 
140  degrees  of  heat  have  been  absorbed  by  the  water.  When,  on  the  contrary, 
water  passes  into  ice,  a  largo  amount  of  heat  which  was  before  latent  in  the 
water,  passes  out  of  it,  and  becomes  sensible. \ 

485.  Another  important  source  of  heat  is 


How  is  me- 
chanical action 
a  source  of  heat  ? 


chanic.ii  action     mechanical  action,   heat  being  produced   by 


friction  and  by  the  condensation,  or  compres- 
sion of  matter.      \ 

What  are  illus-  Savage  nations  kindle  a  fire  by  the  friction  of  two  pieces 
trations  of  the  of  dry  wood ;  the  axles  of  wheels  revolving  rapidly  frequently 
EeatUby°nfric-  become  ignited ;  and  in  the  boring  and  turning  of  metal  the 
tion?  chisels  often  become  intensely  hot.  In  all  these  cases  the 

friction  of  the  surfaces  of  wood  or  of  metal  in  contact,  dis- 
turbs the  latent  heat  of  these  substances,  and  renders  it  sensible. 

The  following  interesting  experiment  was  made  by  Count  Rumford,  to  il- 

*  Latent,  from  the  Latin  word  Inteo,  to  be  hid. 

t  The  phenomena  of  latent  heat  are  further  considered  under  the  head  of  liquefaction. 


214  WELLSIS  NATURAL  PHILOSOPHY. 

lustrate  the  effect  of  friction  in  producing  heat : — A  borer  was  made  to  re- 
volve in  a  cylinder  of  brass,  partially  bored,  thirty-two  times  in  a  minute. 
The  cylinder  was  inclosed  in  a  box  containing  18  pounds  of  water,  the  tem- 
perature of  which  was  at  first  60°,  but  rose  in  an  hour  to  107°;  and  in 
two  hours  and  a  half  the  water  boiled. 

Is  air  necessary  ^ir  does  not  aPPear  to  be  necessary  to  the  production  of 
for  the  produc-  heat  by  the  friction  of  solid  bodies ;  since  heat  is  produced 

frktion1?6*'  by      ty  friction  within  a  vacuum. 

To  whatever  extent  the  operation  may  be  carried,  a  body 
never  ceases  to  give  out  heat  by  friction,  and  this  fact  is  considered  as  a 
strong  argument  in  favor  of  the  theory  that  heat  is  not  a  substance,  but 
merely  a  property  of  matter. 

It  was  formerly  supposed  that  solids  alone  could  develop  heat  by  friction, 
but  recent  experiments  have  proved,  beyond  a  doubt,  that  heat  is  also  gener- 
ated by  the  friction  of  fluids. 

The  heat  excited  by  friction  is  not  in  proportion  to  the  hardness  or  elas- 
ticity of  the  bodies  employed ;  on  the  contrary,  a  piece  of  brass  rubbed  with 
a  piece  of  cedar- wood  produced  more  heat  than  when  rubbed  with  another 
piece  of  metal ;  and  the  heat  was  still  greater  when  two  pieces  of  wood  were 
employed. 

The  reduction  of  matter  into  a  smaller  compass  by  an  exter- 
nal  or  meclianical  force»  is  generally  attended  with  an  evolu- 
production  of  tion  of  heat.  To  such  an  act  of  compression  we  apply  the 
demotion  ?C°n  term  condensation. 

Heat  may  be  evolved  from  air  by  condensation.  This  may 
be  shown  by  placing  a  piece  of  tinder  in  a  tube,  and  suddenly  compressing  the 
air  contained  in  it  by  means  of  a  piston.  The  air  being  thus  condensed,  parts 
with  its  latent  heat  in  sufficient  quantity  to  set  fire  to  the  tinder  at  the  bot- 
tom of  the  tube.  Another  familiar  experiment  of  the  evolution  of  heat  by 
condensation,  is  the  rendering  of  iron  hot  by  striking  it  with  a  hammer.  The 
particles  of  the  iron  being  compressed  by  the  hammer,  can  no  longer  contain 
so  much  heat  in  a  latent  state  as  they  did  before:  some  of  it  therefore  be- 
comes sensible,  and  increases  the  temperature  of  the  metal,  and  the  striking 
may  be  continued  to  such  an  extent  as  to  render  the  iron  red-hot. 

When  a  match  is  drawn  over  sand-paper  or  other  rough  substance,  cer- 
tain phosphoric  particles  are  rubbed  off,  and  being  compressed  between  the 
match  and  the  paper,  their  heat  is  raised  sufficiently  high  to  ignite  them  and 
fire  the  match.  If  the  match  be  drawn  over  a  smooth  surface,  the  compres- 
sion must  be  increased,  for  tiie  temperature  of  the  whole  phosphoric  mass  must 
be  raised  in  order  to  cause  ignition. 

The  fulminating  substance  of  a  percussion-cap  explodes  when  struck  by  a 
hammer,  because  the  blow  occasions  a  compression  of  the  particles,  by  which 
a  sufficient  amount  of  latent  heat  is  liberated  to  produce  ignition. 

what  is  meant ~jS4,86.  Most  living  animals  possess  the  property 
by  vital  heat  ?  0£  maintaining  in  their  system  an  equable  tern- 


SOURCES  OF   HEAT.  215 

perature,  whether  surrounded  by  bodies  that  are  hotter  or 
colder  than  they  are  themselves.  The  cause  of  this  is  due 
to  the  action  of  vital  heat,  or  the  heat  generated  or  ex- 
cited by  the  organs  of  a  living  structure. 

The  following  facts  illustrate  this  principle : — The  explorers  of  the  Arctic 
regions,  during  the  polar  winter,  while  breathing  air  that  froze  mercury,  still 
had  in  them  the  natural  warmth  of  98°.  Fahrenheit  above  zero;  and  the 
inhabitants  of  India,  where  the  same  thermometer  sometimes  stands  at 
115°  in  the  shade,  have  their  blood  at  no  higher  temperature.  Again, 
the  temperature  of  birds  is  not  that  of  the  atmosphere,  nor  of  fishes  that 
of  the  sea. 

487.  The  cause  of  animal  heat  is  undoubt- 
caud  o?  ritai    edlv  due  to  chemical  action  : — the  result  of 

heat?  J.      J  .,   ' 

respiration  and  nervous  excitation. 

Do  lants  os  Growing  vegetables  and  plants  also  possess,  in  a  degree, 

sess  this  prop-  the  property  of  maintaining  a  constant  temperature  within 
erty?  their  structure.  The  sap  of  trees  remains  unfrozen  when  the 

temperature  of  the  surrounding  atmosphere  is  many  degrees  below  the  freez- 
ing point  of  water. 

This  power  of  preserving  a  constant  temperature  in  the  animal  structure  is 
limited.  Intense  cold  suddenly  coming  upon  a  man  who  has  not  sufficient 
protection,  first  causes  a  sensation  of  pain,  and  then  brings  on  an  almost  irre- 
sistible sleepiness,  which  if  indulged  in  proves  fatal.  A  great  excess  of  heat 
also  can  not  long  be  sustained  by  the  human  system. 

Each  species  of  animal  and  vegetable  appears  to  have  a  temperatare  natural 
and  peculiar  to  itself,  and  from  this  diversity  different  races  are  fitted  for  dif- 
ferent portions  of  the  earth's  surface.  Thus,  the  orange-tree  and  the  bird  of 
Paradise  are  confined  to  warm  latitudes;  the  pine-tree  and  the  Arctic  bear, 
to  those  which  are  colder 

When  animals  and  plants  are  removed  from  their  peculiar  and  natural  dis- 
tricts to  one  entirely  different,  they  cease  to  exist,  or  change  their  character 
in  such  a  way  as  to  adapt  themselves  to  the  climate.  As  illustrations  of  this, 
we  find  that  the  wool  of  the  northern  sheep  changes  in  the  tropics  to  a  spe- 
cies of  hair.  The  dog  of  the  torrid  zone  is  nearly  destitute  of  hair.  Bees 
transported  from  the  north  to  the  region  of  perpetual  summer,  cease  to  lay  up 
stores  of  honey,  and  lose  in  a  great  measure  their  habits  of  industry.  .r,t  «a 

Man  alone  is  capable  of  living  in  all  climates,  and  of  migrating  freely  to  all 
portions  of  the  earth.  . 

Of  all  animals,  birds  have  the  highest  temperature ;  mammalia,  or  those 
which  suckle  their  young,  come,  next;  then  amphibious  animals,  fishes,  and 
certain  insects.  Shell-fish,  worms,  and  the  like,  stand  lowest  in  the  scale  of- 
temperature.  The  common  mud-wasp,  in  its  chrysalis  state,  remains  unfrozen, 
during  the  most  severe  cold  of  a  northern  winter ;  the  fluids  of  the  body  in- 
stantly congeal,  however,  in  a  freezing  temperature,  the  moment  the  case, 
or  shell  which  incloses  it,  is  crushed. 


216  WELLS'S   NATURAL   PHILOSOPHY. 


SECTION"    II. 

COMMUNICATION     OF     HEAT. 

488.  Heat  may  be  communicated  in  three 
cated?             ways:  by  CONDUCTION,  by  CONVECTION,  and 
by  RADIATION. 

489.  Heat  is  communicated  by  conduction 

How    is    heat  .  J      . 

communicated     when  it  travels  irom  particle  to  particle  oi  the 

by  conduction?  -  .        _ A  . 

substance,  as  from  the  end  of  the  iron  bar 
placed  in  the  fire  to  that  part  of  the  bar  most  remote 
from  the  fire, 
what  is  con-        490.   When  heat  is  communicated  by  being 

carried  by  the  natural  motion  of  a  substance 
containing  it  to  another  substance  or  place,  as  when  hot 
water  resting  upon  the  bottom  of  a  kettle  rises  and  carries 
heat  to  a  mass  of  water  through  which  it  ascends,  the  heat 
is  said  to  be  communicated  by  convection, 
what  is  radia-  491.  Heat  is  communicated  by  radiation 

tionofheat?         w]ien  ft  \eBipSj  ag  ft  were?  from  a   hot  to  a  Cold 

body  through  an  appreciable  interval  of  space  ;  as  when  a 
body  is  warmed  by  placing  it  before  a  fire  removed  to  a 
little  distance  from  it. 

now  does  a  492.  A  heated  body  cools  itself,  first  by  giv- 
cooMtseifb?ody  *no  °ff  heat  from  i*s  surface,  either  by  conduc- 
tion or  radiation,  or  both  conjointly  ;  and  sec- 
ondly, by  the  heat  in  its  interior  passing  from  particle  to 
particle  by  conduction,  through  its  substance  to  the  sur- 
face. A  cold  body,  on  the  contrary,  becomes  heated  by  a 
process  directly  the  reverse  of  this. 

DO  an  bodies  493.  Different  bodies  exhibit  a  very  great 
SSSiy* weii?at  degree  of  difference  in  the  facility  with  which 
they  conduct  heat :  some  substances  oppose 
very  little  impediment  to  its  passage,  while  through  others 
it  is  transmitted  slowly. 

what  are  con-  494.  All  bodies  are  divided  into  two  classes 
nonconductor  m  Aspect  to  their  conduction  of  heat,  viz., 
of  he»t?  m^0  conductors  and  non-conductors.  The  for- 


COMMUNICATION   OF    HEAT. 


217 


FIG-  198. 


mer  are  such  as  allow  heat  to  pass  freely  through  them  ; 
the  latter  comprise  those  which  do  not  give  an  easy  pass- 
age to  it. 

Dense  solid  bodies,  like  the  metals,  are  the  best  con- 
ductors of  heat  ;°  light,  porous  substances,  more  especially 
those  of  a  fibrous  nature,  are  the  worst  conductors  of  heat. 

The  different  conducting  power  of  various  solid  substances  may  be  strik- 
ingly shown  by  taking  a  series  of  rods  of  equal  length  and  thickness,  coating 
one  of  their  extremities  with  wax,  and  placing  the  other  extremities  equally 
in  a  source  of  heat.  The  wax  will  be  found  to  entirely  melt  off  from  some 
of  the  rods  before  it  has  hardly  softened  upon  others. 

what  is  the  495.  Liquids  are  almost 
rowderctofsiiq-  absolute  non-conductors  of 
uids?  heat. 

If  a  small  quantity  of  alcohol  be  poured  on  the  sur- 

face of  water  and  inflamed,  it  will  continue  to  bum 

for  some  time.      (See  Fig.   198.)    A  thermometer, 

immersed  at  a  small  depth  below  the  common  sur- 

face of  the  spirit  and  the  water,  will  fail  to  show  any 

Increase  in  temperature. 

Another  and  more  simple  experiment  proves  the 

same  fact  ;  as  when  a  blacksmith  immerses  his  red- 

hot  iron  in  a  tank  of  water,  the  water  which  sur- 

rounds the  iron  is  made  boiling  hot,  while  the  water 

not  immediately  in  contact  with  it  remains  quite  cold. 

FIG.  199.  If  a  tube  nearly  filled  with  water  is  held 

over  a  spirit  lamp,  as  in  Fig.  199,  in  such  a 
manner  as  to  direct  the  flame  against  the 
upper  layers  of  the  water,  the  water  will  bo 
observed  to  boil  at  the  top,  but  remain  cool 
below.  If  quicksilver,  on  the  contrary,  be 
so  treated,  its  lower  layers  will  speedily  be- 

come heated.     The  particles  of  mercury  will  communicate  the  heat  to  each 

other,  but  the  particles  of  water  will  not  do  so. 

A  stone,  or  marble  hearth  in  an  apartment,  feels  colder  to 
Why    does    a 
stone,  or  mar-      the  feet  than  a  woolen  carpet,  or  hearth-rug,  not  because  the 

colderthaifa  °n6  is  hotter  than  the  other»  for  both  are  reall7  of  the  same 
carpet?  temperature,  but  because  the  stone  and  marble  are  good 

*  The  following  table  exhibits  the  relative  conducting  power  of  different  substances, 
the  ratio  expressing  the  conducting  power  of  gold  being  taken  at  100  ; 


Gold 
Platinum 
Silver  . 
Copper  . 
Iron  , 
Zioe 


100-00 
98-10 
97-30 


37-41 
36-3T 


Tin 30-38 

Lead 17-96 

Marble  .  .  .  .  2-34 
Porcelain  .  .  .  .1-22 
Brick  earth  ,  .  .  1-13 


10 


218  WELLS'S   NATURAL   PHILOSOPHY. 

conductors,  and  the  woolen  carpet  and  hearth-rug  very  bad  conductors. 
The  action  of  the  two  substances  is  as  follows: — As  soon  as  the  hcaith- 
stone  has  absorbed  a  portion  of  heat  from  the  feet,  it  instantly  disposes 
of  it  by  conducting  it  away,  and  calls  for  a  fresh  supply  ;  and  this  ac- 
tion will  continue  until  the  stone  and  the  foot  have  established  an  equili- 
brium of  temperature  between  them.  The  carpet  and  the  rug  also  absorb 
heat  from  the  feet  in  like  manner,  but  they  conduct  or  convey  it  away  so 
slowly,  that  its  loss  is  hardty  perceptible. 

Most  varieties  of  wood  are  bad  conductors  of  heat ;  hence,  though  one  end 
of  a  stick  is  blazing,  the  other  end  may  be  quite  cold.  Cooking  vessels,  for 
this  reason,  are  often  furnished  with  wooden  handles,  which  conduct  the  heat 
of  the  vessel  too  slowly  to  render  its  influx  into  our  hands  painful.  For  tho 
same  reason  we  use  paper  or  woolen  kettle-holders. 

TO  what  extent  496.  Bodies  in  the  gaseous,  or  aeriform  con- 
bodiesrco°ndict  dition  are  more  imperfect  conductors  of  heat 
beat?  than  liquids.  Common  air,  especially,  is  one 

of  the  worst  conductors  of  heat  with  which  we  are  ac- 
quainted. 

HOW  is  air  497.  Air  is,  however,  readily  heated  hy  con- 

vection. Thus,  when  a  portion  of  air  by  com- 
ing in  contact  with  a  heated  body  has  heat  imparted  to 
it,  it  expands,  and  becoming  relatively  lighter  than  the 
other  portions  around  it,  rises  upward  in  a  current,  carry- 
ing the  heat  with  it  ;  other  colder  air  succeeds,  and  (being 
heated  in  a  similar  way)  ascends  also.  A  series  of  cur- 
rents are  thus  formed,  which  are  called  "  convective  cur- 
rents." 

In  this  way  air,  which  is  a  bad  conductor,  rapidly  reduces  the  temperature 
of  a  heated  substance.  If  the  air  which  encases  the  heated  substance  were 
to  remain  perfectly  motionless,  it  would  soon  become,  by  contact,  of  the  same 
temperature  as  the  body  itself,  and  the  withdrawal  of  heat  would  be  checked  ; 
but  as  the  external  air  is  never  perfectly  at  rest,  fresh  and  colder  portions 
continually  replace  and  succeed  those  which  have  become  in  any  degree 
heated,  and  thus  the  abstraction  of  heat  goes  on. 

For  this  reason  a  windy  day  always  feels  colder  than  a  calm  day  of  the 
same  temperature,  because  in  the  former  case  the  particles  of  air  pass  over 
us  more  rapidly,  and  every  fresh  particle  takes  some  portion  of  heat. 

HOW  may  the  498.  Tne  conducting  power  of  all  bodies  is 
pow^o&ies  diminished  by  pulverizing  them,  or  dividing 
Se diminished?  them  into  fine  filaments. 

Thus  saw-dust,  when  not  too  much  compressed^  is  one  of  the  most  perfect 


COMMUNICATION   OF   HEAT.  219 

non-conductors  of  heat.    "Wool,  fur,  -hair  and  feathers,  are  also  among  the  worst 
conductors  of  heat. 

r  furs  Woolens  and  ftirs  are  used  for  clothing  in  cold  weather,  not 
and  wooiena  because  they  impart  any  heat  to  the  body,  but  because  they 
used  for  cloth-  are  veiy  ^^  conductors  of  heat ;  and  therefore  prevent  the 
warmth  of  the  body  from  being  drawn  off  by  the  cold  air. 
The  heat  generated  in  the  animal  system  by  vital  action  has  constantly  a 
tendency  to  escape,  and  be  dissipated  at  the  surface  of  the  body,  and  the  rate 
at  which  it  is  dissipated  depends  on  the  difference  between  the  temperature 
of  the  surface  of  the  body  and  the  temperature  of  the  surrounding  medium. 
By  interposing,  however,  a  non-conducting  substance  between  the  surface  of 
the  body  and  the  external  atmosphere,  we  prevent  the  loss  of  heat  which 
would  otherwise  take  place  to  a  greater  or  less  degree. 

The  non-conducting  properties  of  fibrous  and  porous  sub- 

Ui°    W^K>Q  con9    s^ances  are  due  almost  altogether  to  the  air  contained  in  their 

ducting     prop-   interstices,  or  between  their  fibers.     These  are  so  disposed  as 

Bubstaucesdue  ?   *°  rece^ve  an(i  retain  a  large  quantity  of  air  without  permitting 

it  to  circulate. 

The  warmest  clothing  is  that  which  fits  the  body  rather  loosely,  because  more 
hot  air  will  be  confined  by  a  moderately  loose  garment  than  by  one  which  fits 
the  body  tightly. 

Blankets  and  warm  woolen  goods  are  always  made  with  a  nap  or  projec- 
tion of  fibers  upon  the  outside,  in  order  to  take  advantage  of  this  principle. 
The  nap  or  fibers  retain  air  among  them,  which,  from  its  non-conducting 
properties,  serves  to  increase  the  warmth  of  the  material. 

The  finer  the  fibers  of  hair,  or  wool,  the  more  closely  they 
ence  has  the  retain  the  air  enveloped  within  them,  and  the  more  imperme- 
fibers  IT  on  the  able  they  become  to  neat-  In  accordance  with  this  princi- 
warmth  of  a  pie,  the  external  coverings  of  animals  vary  not  only  with  the 
climate  which  the  species  inhabit,  but  also  in  the  same  indi- 
vidual they  change  with  the  season.  In  warm  climates  the  furs  are  generally 
coarse  and  thin,  while  in  cold  countries  they  are  fine,  close,  light,  and  of  uni- 
form texture,  almost  perfect  non-conductors  of  heat. 

We  have  illustrations  of  this  principle  also  in  the  vegetable  kingdom.  The 
bark  of  trees,  instead  of  being  compact  and  hard  like  the  wood  it  envelops, 
is  porous  and  formed  of  fibers,  or  "layers,  which,  by  including  more  or  less  of 
air  between  their  surfaces,  are  rendered  non-conductors,  and  prevent  the 
escape  of  heat  from  the  body  of  the  tree. 

An  apartment  is  rendered  much  warmer  for  being  furnished  with  double 
doors  and  windows,  because  the  air  confined  between  the  two  surfaces  op- 
poses both  the  escape  of  heat  from  within,  and  the  admission  of  cold  from 
•without. 

As  a  non-conducting  substance  prevents  the  escape  of  heat  from  within  a 
body,  so  it  is  equally  efficacious  in  preventing  the  access  of  heat  from  without. 
In  an  atmosphere  hotter  than  our  bodies,  the  effect  of  clothing  would  be  to 
keep  the  body  cool  Flannel  is  one  of  the  warmest  articles  of  dress,  yet  we 


220  WELLS'S   NATURAL   PHILOSOPHY. 

can  not  preserve  ice  more  effectually  in  summer  than  by  enveloping  it  in  its 
folds.  Firemen  exposed  to  the  intense  heat  of  furnaces  and  steam-boilers,  in- 
variably protect  themselves  with  flannel  garments. 

Cargoes  of  ice  shipped  to  the  tropics,  are  generally  packed  for  preservation 
in  sawdust:  a  casing  of  sawdust  is  also  one  of  the  most  effectual  means  of 
preventing  the  escape  of  heat  from  the  surfaces  of  steam-boilers  and  steam- 
pipes.  Straw,  from  its  fibrous  character,  is  an  excellent  non-conductor  of 
heat,  and  is  for  this  reason  extensively  used  by  gardeners  for  incasing  plants 
and  trees  which  are  exposed  to  the  extreme  cold  of  winter. 

Snow  protects  the  soil  in  winter  from  the  effects  of  cold  in 
How  does  snow 

protect  the  the  same  way  that  far  and  wool  protect  animals,  and  cloth- 
earth  from  mg  man>  gnow  is  ma(ie  Up  Of  an  infinite  number  of  little 
crystals,  which  retain  among  their  interstices  a  large  amount 
of  air,  and  thus  contribute  to  render  it  a  non-conductor  of  heat.  A  covering 
of  snow  also  prevents  the  earth  from  throwing  off  its  heat  by  radiation.  The 
temperature  of  the  earth,  therefore,  when  covered  with  snow,  rarely  descends 
much  below  the  freezing-point,  even  when  the  air  is  fifteen  or  twenty  de- 
grees colder.*  Thus  roote  and  fibers  of  trees  and  plants,  are  protected  from  a 
destructive  cold. 

499.  Clothing  is  considered  warm  or  cool  ac- 

Under      what  .      .  ,  .,. 

circumstances     cording  as  it  impedes  or  facilitates  the  passage 

is  clothing  con-  ,.  c  #  i      j  • 

side-red  warm     of  heat  to  or  from  the  surface  of  our  bodies. 
The  finer  the  cloth,  the  more  slowly  it  con- 
ducts  heat.      Fine  cloths,  therefore,   are    warmer   than 
coarse  ones. 

Woolen  substances  are  worse  conductors  of  heat  than  cotton,  cotton  than 
silk,  and  silk  than  linen.  A  flannel  shirt  more  effectually  intercepts  heat 
than  cotton,  and  a  cotton  than  a  linen  one. 

The  sheets  of  a  bed  feel  colder  than  the  blankets,  because  they  are  better 
conductors  of  heat,  and  carry  off  the  heat  more  rapidly  from  the  body,  the 
actual  temperature  of  both,  however,  is  the  same.  For  the  same  reason,  a 
linen  handkerchief  is  cooler  and  more  agreeable  to  the  face  than  a  cotton  one. 

Cellars  feel  cool  in  summer,  and  warm  in  winter,  because  the  external  air 

*  "  Few  can  realize  the  protecting  value  of  the<warm  coverlet  of  snow.  No  eider-down 
in  the  cradle  of  an  infant  is  tucked  in  more  kindly  than  the  sleeping-dress  of  winter  about 
the  feeble  flower-life  of  the  Arctic  regions.  The  first  warm  snows  of  August  and  Septem- 
ber, falling  on  a  thickly-blended  carpet  of  grasses,  heaths  and  willows,  enshrine  the 
flowery  growths  which  nestle  around  them  in  a  non-conducting  air-chamber ;  and  as 
each  successive  snow  increases  the  thickness  of  the  cover,  we  have,  before  the  intense  cold 
of  winter  sets  in,  a  light  cellular  bed,  covered  by  drift,  six,  eight,  or  ten  feet  deep,  in 
which  the  plant  retains  its  vitality.  The  frozen  sub-soil  does  not  encroach  upon  this  nar- 
row cover  of  vegetation.  I  have  found,  in  mid-winter,  in  the  high  latitude  of  78°,  the 
surface  so  nearly  moist  as  to  be  friable  to  the  touch ;  and  on  the  ice-floes  commencing 
with  a  surface  temperature  of  30°  below  zero,  I  found,  at  two  feet  deep,  a  temperature  of 
8*  below  zero,  at  four  feet  2°  above  zero,  and  at  eight  feet  26°  above  zero.  My  experi- 
ments prove  that  the  conducting  power  of  snow  is  proportioned  to  its  compression  by 
winds,  rains,  drifts,  and  congelation." — DB.  KANE'S  Second  Arctic  Expedition. 


COMMUNICATION   OF   HEAT. 


221 


Upon  what 
principle  are 
refrigerators 
and  fire-proof 
safes  con- 
structed ? 


has  not  free  access  into  them ;  in  consequence  of  which  they  remain  almost  at 
an  even  temperature,  which  in  summer  is  about  10  degrees  colder,  and  in 
winter  about  10  degrees  warmer  than  the  external  air. 

500.  Kefrigerators,  used  for  the  preservation  of  animal  and 
vegetable  substances  in  warm  weather,  are  double-walled 
boxes,  with  the  spaces  between  the  sides  filled  with  powdered 
charcoal,  or  some  other  porous,  non-conducting  substance. 

The  so-called  "fire-proof"  safes  are  also  constructed  of 
double  or  treble  walls  of  iron,  with  intervening  spaces  between  them  filled 
with  gypsum,  or  "  Plaster  of  Paris."  This  lining,  which  is  a  most  perfect 
non-conductor,  prevents  the  heat  from  passing  from  the  exterior  of  the  safe 
to  the  books  and  papers  within.  The  idea  of  applying  "  Plaster  of  Paris"  in 
this  way  for  the  construction  of  safes,  originated,  in  the  first  instance,  from  a 
workman  attempting  to  heat  water  in  a  tin  basin,  the  bottom  and  sides  of 
which  were  thinly  coated  with  this  substance.  The  non-conducting  proper- 
ties of  the  plaster  were  so  great  as  to  almost  entirely  intercept  the  passage  of 
the  heat,  and  the  man,  to  his  surprise,  found  that  the  water,  although  directly 
over  the  fire,  did  not  get  hot. 

501.  It  has  been  already  stated  that  liquids  and  gases 
are  non-conductors  of  heat,  and  can  not  well  be  heated, 
like  a  mass  of  metal,  or  any  solid,  by  the  communication 
of  heat  from  particle  to  particle. 

This  peculiarity  is  owing  to  the  mobility 
which  subsists  among  the  particles  of  all  fluids, 
and  to  the  change  in  the  size  of  the  particles, 
which  is  invariably  produced  by  a  change  in 
their  temperature. 

The  constituent  particles  of  solid  bodies  being  incapable  of  changing  their 
relative  position  and  arrangement,  the  heat  can  only  pass  through  them,  from 
particle  to  particle,  by  a  slow  process ;  but  when  the  particles  forming  any 
stratum  of  liquid  are  heated,  their  mass,  expanding,  becomes 
lighter,  bulk  for  bulk,  than  the  colder  stratum  immediately 
above  it,  and  ascends,  allowing  the  superior  strata  to  descend. 

HOW  is  water  502.  When  the  heat  enters  at 
made  hot?  tke  bottom  of  a  vessel  containing 
water,  a  double  set  of  currents  is  immediately 
established— one  of  hot  particles  rising  to- 
ward the  surface,  and  the  other  of  colder  par- 
ticles descending  to  the  bottom.  The  por- 
tion of  liquid  which  receives  heat  from  below 
is  thus  continually  diffused  through  the  other 


Why  can  not 
liquids  and 
gases  be  heat- 
ed in  the  same 
manner  as  sol- 
ids? 


FIG.  200. 


222  WELLS'S  NATURAL  PHILOSOPHY. 

parts,  and  the  heat  is  communicated  by  the  motion  of  the 
particles  among  each  other. 

These  currents  take  place  so  rapidly,  that  if  a  thermometer  be  placed  at 
the  bottom  and  another  at  the  top  of  a  long  jar,  the  fire  being  applied  below, 
the  upper  one  will  begin  to  rise  almost  as  soon  as  the  lower  one.  The 
movement  of  the  particles  of  water  in  boiling  will  be  understood  by  reference 
to  Fig.  200.  They  may  be  rendered  visible  by  adding  to  a  flask  of  boiling 
water  a  few  small  particles  of  bituminous  coal,  or  flowers  of  sulphur. 

Air  and  other  gases  are  heated  in  precisely  the  same  manner  as  water, 
and  this  method  of  communicating  heat  is  termed  convection. 

Heat,  however,  passes  by  conduction  between  the  particles  of  both  liquids 
and  gases,  but  to  such  a  slight  extent,  that  they  were  for  a  long  time  re- 
garded as  entirely  incapable  of  conducting  heat. 

in  what  man  503.  The  process  of  cooling  in  a  liquid  is 
copied?  Hquid  directly  the  reverse  of  that  of  heating.  The 
particles  at  the  surface,  by  contact  with  the 
air,  readily  lose  their  heat,  become  heavier,  and  sink,  while 
the  warmer  particles  below  in  turn  rise  to  the  surface. 

To  heat  a  liquid,  therefore,  the  heat  should  be  applied  at  the  bottom  of 
the  mass ;  to  cool  it,  the  cold  should  be  applied  at  the  top,  or  surface. 

The  facility  with  which  a  liquid  may  be  heated  or  cooled  depends  in  a  great 
degree  on  the  mobility  of  its  particles.  "Water  may  be  made  to  retain  its 
heat  for  a  long  time  by  adding  to  it  a  small  quantity  of  starch,  the  particles 
of  which,  by  thei*  viscidity  or  tenacity,  prevent  the  free  circulation  of  the 
heated  particles  of  water.  For  the  same  reason  soup  retains  its  heat  longer 
than  water,  and  all  thick  liquids,  like  oil,  molasses,  tar,  etc.,  require  a  consid- 
erable time  for  cooling. 

E  504.  When  the  hand  is  placed  near  a  hot  body  suspended 

phenomena  of  in  the  air,  a  sensation  of  warmth  is  perceived,  even  for  a 
radiation .  considerable  distance.  If  the  hand  be  held  beneath  the  body, 

the  sensation  will  be  as  great  as  upon  the  sides,  although  the  heat  has  to 
shoot  down  through  an  opposing  current  of  air  approaching  it.  This  effect 
does  not  arise  from  the  heat  being  conveyed  by  means  of  a  hot  current,  since 
all  the  heated  particles  have  a  uniform  tendency  to  rise ;  neither  can  it  de- 
pend upon  the  conducting  power  of  the  air,  because  aeriform  substances  pos- 
sess that  power  in  a  very  low  degree,  while  the  sensation  in  the  present  case 
is  excited  almost  on  the  instant.  This  method  of  distributing  heat,  to  dis- 
tinguish it  from  heat  passing  by  conduction,  or  convection,  is  called  radiation, 
and  heat  thus  distributed  is  termed  radiant,  or  radiated  heat. 

,  .  _.          505.  All  bodies  radiate  heat  in  some  meas- 

Do  all  bodies 

radiate    heat    tire,  but  not  all  equally  well ;  radiation  being 

equally  well?  3  .  11  n  • 

in  proportion  to  the  roughness  oi  the  radiating 
surface.     All  dull  and  dark  substances  are,  for  the  most 


COMMUNICATION   OF   HEAT.  223 

part,  good  radiators  of  heat ;  but  bright  and  polished  sub- 
stances are  generally  bad  radiators.0  Color,  however, 
alone,  has  no  effect  on  the  radiation  of  heat. 

If  a  metal  surface  be  scratched,  its  radiating  power  is  increased.  A  liquid 
contained  in  a  bright,  highly-polished  metal  pot,  will  retain  its  heat  much 
longer  than  in  a  dull  and  blackened  one.  This  is  not  due  to  the  polish  or 
brightness  of  the  surface,  but  to  the  fact  that,  by  polishing,  the  surface  is  ren- 
dered dense  and  smooth,  and  such  surfaces  do  not  allow  the  heat  to  escape 
readily.  If  we  cover  the  polished  metal  surface  with  a  thin  cotton  or  linen 
cloth,  so  as  to  render  the  surface  less  dense,  the  radiation  of  heat,  and  conse- 
quent cooling,  will  proceed  rapidly. 

Black  lead  is  one  of  the  best  known  radiators  of  heat,  and  on  this  account 
is  generally  employed  for  the  blackening  of  stoves  and  hot-air  flues.  As  a 
high  polish  is  unfavorable  to  radiation,  stoves  should  not  be  too  highly  polished 
with  this  substance. 

Heat  radiated  from  the  sun  is  all  radiant  heat.     J 

506.  Heat  is  propagated  through  space  by 
propagated  by    radiation  in  straight   lines,   and  its  intensity 

radiation?  .  ,.          ,  ^    ,  '  ,  .    ,  J 

varies  according  to  the  same  law  which  governs 
the  attraction  of  gravitation,  that  is,  inversely  as  to  the 
square  of  the  distance.f 

Thus  the  heating  effects  of  any  hot  body  is  nine  times  less  at  three  feet  than 
at  one ;  sixteen  times  less  at  four  feet ;  and  twenty-five  times  less  at  five. 

The  velocity  with  which  radiant  heat  moves  through  space 
Ey  does*  ra-      is>  in  a11  probability,  the  same  as  the  velocity  of  light.    Some 
tiiant        heat      authorities,  however,  consider  it  to  be  only  four  fifths  of  that 
of  light,  or  about  164,000  miles  in  a  second  of  time. 

Does  radiation  507.  The  radiation  of  heat  goes  on  at  all 
sta0nteiydfrom°rii  times,  and  from  all  surfaces,  whether  their 
temperature  be  the  same  or  different  from 
that  of  surrounding  objects  :  therefore  the  temperature 
of  a  body  falls  when  it  radiates  more  heat  than  it  absorbs; 
its  temperature  is  stationary  when  the  quantities  emitted 
and  received  are  equal ;  and  it  grows  warm  when  the  ab- 
sorption exceeds  the  radiation. 

*  The  action  of  a  blackened  surface  of  tin  being  assumed  as  100,  it  has  been  found  that 
that  of  a  steel  plate  was  15 ;  of  clean  tin,  1 2 ;  of  tin  scraped  bright,  16 ;  when  scraped  with 
the  edge  of  a  fine  file  in  one  direction,  26;  when  scraped  again  across,  about  13;  a  sur- 
face of  clean  lead,  19 ;  covered  with  a  gray  crust,  45  ;  a  thin  crust  of  isinglass,  80  ;  rosin 
96 ;  writing-paper,  98  ;  ice.  85. 

t  It  is  an  exceedingly  curious  fact  that  this  law  applies  to  all  physical  influences  that 
spread  from  a  center,  such  as  gravitation,  heat,  light,  electricnl  forces,  magnetism  and 
sound ;  and  to  all  central  forces,  when  not  weakened  by  any  resistance  or  opposing  force. 


224  WELLS'S  NATUKAL  PHILOSOPHY. 

If  a  body,  at  any  temperature,  be  placed  among  other  bodies,  it  will  affect 
their  condition  of  temperature,  or  as  we  express  it,  thermally ;  just  as  a  candle 
brought  into  a  room  illuminates  all  bodies  in  its  presence ;  with  this  difference, 
however,  that  if  the  candle  be  extinguished,  no  more  light  is  diffused  by  it ; 
but  no  body  can  be  thermally  extinguished.  All  bodies,  however  low  be 
their  temperature,  contain  heat,  and  therefore  radiate  it. 

If  a  piece  of  ice  be  held  before  a  thermometer,  it  will  cause 

the  mercury  m  its  tube  to  fall,  and  hence  it  has  been  sup- 
sink  when  posed  that  the  ice  emitted  rays  of  cold.  This  supposition  is 
?cre°"ght  near  erroneous.  The  ice  and  the  thermometer  both  radiate  heat, 

and  each  absorbs  more  or  less  of  what  the  other  radiates  to- 
ward it.  But  the  ice,  being  at  a  lower  temperature  than  the  thermometer, 
radiates  less  than  the  thermometer,  and  therefore  the  thermometer  absorbs 
less  than  the  ice,  and  consequently  falls.  If  the  thermometer  placed  in  the 
presence  of  the  ice  had  been  at  a  lower  temperature  than  the  ice,  it  would, 
for  like  reasons,  have  risen.  The  ice  in  that  case  would  have  warmed  the 
thermometer. 

What  do  we  509'  Radiations>  or  effects  which  are  propagated  in  straight 
mean  by  rays  lines  only  (such  as  light  and  radiant  heat),  are  most  conve- 
Ught?eat  °r  m'ently  considered  by  dividing  them  into  innumerable  straight 

lines,  or  rays ;  not  that  there  are  any  such  divisions  in  nature, 
but  they  enable  us  more  readily  to  comprehend  the  nature  of  the  phenomena 
with  which  these  principles  are  concerned. 

when  radiant  510.  When  rays  of  heat  radiated  from  one 
^dy  fall  upon  the  surface  of  another  body,  they 
maybe  disposed  of  in  three  ways :  I.They 

posed  of?          may  re|)OUnci  from  its  surface,  or  be  reflected  ; 

2.  They  may  be  received  into  its  surface,  or  be  absorbed  ; 

3<  They  may  pass  directly  through  the  substance  of  the 

body,  or  be  transmitted. 

511.  A  ray  of  heat  radiated  from  the  sur- 
face  of  a  body  proceeds  in  a  straight  line  until 
it  meets  a  reflecting  surface,  from  which  it 

rebounds  in  another  straight  line,  the  direction  of  which 

is  determined  by  the  law  that  the  angle  of  incidence  is 

equal  to  the  angle  of  reflection. 

The  manner  in  which  heat  is  reflected  is  strikingly  shown  by  taking  two 
concave  mirrors,  MandN,  Fig.  201,  of  bright  metal,  about  one  foot  in  diameter, 
and  placing  them  exactly  opposite  to  each  other  at  a  distance  of  about  ten 
feet.  In  the  focus  of  one  mirror,  as  at  A,  is  placed  a  heated  body,  as  a  mass 
of  red  hot  iron,  and  in  the  focus  of  the  other  mirror,  as  at  B,  a  small  quantity 
of  gunpowder,  or  a  piece  of  phosphorus.  The  rays  of  heat,  radiated  in  diverg- 
ing lines  from  the  hot  metal,  strike  upon  the  surface  of  the  mirror  M,  and  are 


COMMUNICATION   OF   HEAT. 


225 


reflected  by  it  in  parallel  lines  to  the  surface  of  the  opposite  mirror,  N,  where 
they  will  be  caused  to  converge  to  its  focus,  B,  and  ignite  the  powder  or 
phosphorus  at  that  point. 

Fia.  201. 


512.   Polished  metallic  surfaces  constitute 
reflectors    of    the  best  reflectors  of  heat ;  but  all  bright  and 
light  colored  surfaces  are  adapted  for  this  pur- 
pose to  a  greater  or  less  degree.* 

Water  requires  a  longer  time  to  become  hot  in  a  bright  tin  vessel  than  hi  a 
dark  colored  one,  because  the  heat  is  reflected  from  the  bright  surface,  and 
does  not  enter  the  vessel. 

HOW  does  the  513.  The  power  of  absorbing  heat  varies 
sorbing0*  heat  w^n  almost  every  form  of  matter.  Surfaces 
are  good  absorbers  of  heat  in  proportion  as 
they  are  poor  reflectors.  The  best  radiators  of  heat  also 
are  the  most  powerful  absorbers,  and  the  most  imperfect 
reflectors. 

Dark  colors  absorb  heat  from  the  sun  more  abundantly  than  light  ones. 
Tin's  may  be  proved  by  placing  a  piece  of  black  and  a  piece  of  white  cloth 
upon  the  snow  exposed  to  the  sun ;  in  a  few  hours  the  black  cloth  will  have 
melted  the  snow  beneath  it,  while  the  white  cloth  will  have  produced  little 
or  no  effect  upon  it. 

The  darker  any  color  is,  the  warmer  it  is,  because  it  is  a  better  absorbent 
of  heat.  The  order  may  be  thus  arranged :  1,  black  (warmest  of  all) ;  2,  violet  ; 
3,  indigo ;  4,  blue ;  5,  green ;  6,  red ;  7,  yellow ;  and  8,  white  (coldest  of  all). 

*  Of  100  rays  falling  at  an  angle  of  60°  from  the  perpendicular,  polished  gold  will  reflect 
76  ,•  silver  62 ;  brass,  62 ;  brass  without  polish,  52  ;  polished  b'fass  varnished,  41 ;  looking- 
glass,  20 ;  glass  plate  blackened  on  the  back,  12 ;  and  metal  plate  blackened,  0. 

io* 


226  WELLS'S  NATURAL  PHILOSOPHY. 

A  piece  of  brown  paper  submitted  to  the  action  of  a  burning-glass,  ignites 
much  more  quickly  than  a  piece  of  white  paper.  The  reason  of  this  is,  that 
the  white  paper  reflects  the  rays  of  the  sun,  and  though  but  slightly  heated 
appears  highly  luminous ;  while  the  brown  paper,  which  absorbs  the  rays, 
readily  becomes  heated  to  ignition.  For  the  same  reason  a  kettle  whose  bot- 
tom and  sides  are  covered  with  soot,  heats  water  more  readily  than  a  kettle 
whose  sides  are  bright  and  clean. 

Light-colored  fabrics  are  most  suitable  for  dresses  in  summer,  since  they 
reflect  the  direct  heat  of  the  sun,  and  do  not  absorb  it ;  black  outside  gar- 
ments, on  the  contrary,  are  most  suitable  for  winter,  as  they  absorb  heat 
readily,  but  do  not  reflect  it. 

Hoar-frost,  in  the  spring  and  autumn,  may  be  observed  to  remain  longer 
in  the  presence  of  the  morning  sun,  on  light-colored  substances  than  upon  the 
dark-colored  soil,  etc. ;  the  former  do  not  absorb  the  heat,  as  the  dark-colored 
bodies  do,  but  reflect  it,  and  in  consequence  of  this  they  remain  too  cold  to 
thaw  the  frost  deposited  upon  their  surfaces. 

H      's  th   at  ^^"  "^r  a^sor^s  h6^  Ver7  slowly,  and  does  not  readily 

mosphere  heat-  part  with  it.  Air  rarely  radiates  heat,  and  is  not  heated  to 
ed  ?  great  extent  by  the  direct  rays  of  the  sun,  The  sun,  however, 

heats  the  surface  of  the  earth,  and  the  air  resting  upon  it  is  heated  by  contact 
with  it,  and  ascends,  its  place  being  supplied  by  colder  portions,  which  in  turn 
are  heated  also. 

This  reluctance  of  air  to  part  with  its  heat  occasions  some  very  curious  dif- 
ferences between  its  burning  temperature  and  that  of  other  bodies.  Metals, 
which  are  generally  the  best  conductors,  and  therefore  communicate  heat 
most  readily,  can  not  be  handled  with  impunity  when  raised  to  a  temperature 
of  more  than  120°  F. ;  water  becomes  scalding  hot  at  150°  F. ;  but  air  ap- 
plied to  the  skin  occasions  no  very  painful  sensation  when  its  heat  is  far  be- 
yond that  of  boiling  water.  ^ 

Some  curious  experiments  have  been  made  in  reference  to  the  power  of  the 
human  body  to  withstand  the  influence  of  heated  air.  Sir  Joseph  Banks  en- 
tered an  oven  heated  52°  above  the  boiling  point,  and  remained  there  some 
time  without  inconvenience.  During  the  time,  eggs,  placed  on  "a  metal  frame, 
were  roasted  hard,  and  a  beefsteak  was  overdone.  But  though  he  could 
thus  bear  the  contact  of  the  heated  air,  he  could  not  bear  to  touch  any  metal- 
lic substance,  as  a  watch-chain,  money,  etc.  Workmen,  also,  enter  ovens, 
in  the  manufacture  of  molds  of  plaster  of  Paris,  in  which  the  thermometer 
stands  100°  above  the  temperature  of  boiling  water,  and  sustain  no  injury. 

in  what  man-        515.  Heat,  in  passing  through   most  sub- 

transmitted6**  stances,  or  media,  is  retained,  or  intercepted 
m  *ts  Passage  m  a  greater  or  less  degree.  The 
capacity  of  solids  and  liquids  for  transmitting 

heat  is  not  always  in  proportion  to  their  transparency,  or 

capacity  for  transmitting  light. 


THE   EFFECTS   OF   HEAT.  227 

516.  The  heat  of  the  sun  passes  through  transparent 
bodies  without  loss  ;  but  heat  from  terrestrial  sources  is 
in  great  part  arrested  by  many  substances  which  allow 
light  to  pass  freely,  —  such  as  water,  alum,  glass,  etc. 

Thus,  a  plate  of  glass  held  between  one's  face  and  the  sun  will  not  protect 
it,  but  held  between  the  face  and  a  fire,  it  will  intercept  a  large  proportion  of 
the  heat. 

517.  Those  substances  which  allow  heat  to  pass  freely 
through  them,  are  called  diathermanous,  and  those  which 
retain  nearly  all  the  heat  they  receive,  are  called  ather- 
manous. 

Eock-salt  allows  heat  to  pass  through  it  more  readily  than  any  other  known 
substance  ;  while  a  thin  plate  of  alum,  which  is  nearly  transparent,  almost 
entirely  intercepts  terrestrial  heat.  Heat,  indeed,  will  pass  more  readily 
through  a  black  glass,  so  dark  that  the  sun  at  noon  is  scarcely  discernible 
through  it,  than  through  a  thin  plate  of  clear  alum.  Water  is  one  of  the  least 
diathermanous  substances,  although  its  transparency  is  nearly  perfect.  If, 
therefore,  it  is  desired  to  transmit  light  without  heat,  or  with  greatly  dimin- 
ished heat,  it  is  only  necessary  to  let  the  rays  pass  through  water,  by  which 
they  will  be  strained  of  a  great  part  of  their  heat. 

How  does  the  ^  ^ias  '3een  f°un^  that  the  power  of  heat  to  penetrate  a 
temperature  of  dense,  transparent  substance,  is  increased  in  proportion  as  the 
rns°he7ataadffect  temperature  of  the  body  from  which  it  is  radiated  is  increased. 
its'  ^transmia-  Heat,  also,  accompanied  by  light,  is  transmitted  more  readily 
than  heat  without  light. 

Isara  of  solar  518*  Heat  and  ligllt  come  to  us  conjointly  from,  the  sun. 
heat  simple  or  When  a  ray  of  light  is  caused  to  pass  through  a  prism  it  is 
itsnatur^y  "*  analvzed  or  separated  into~  seven  brilliant  colors,  or  element- 
ary parts.  If  the  heat  ray  which  accompanies  the  light  is 
treated  in  a  similar  manner,  our  organs  of  sight  are  so  constituted  that  we 
ao  not  discover  any  separation  to  have  taken  place  in  it.  It  is,  however,  es- 
tablished beyond  a  doubt,  that  in  the  same  manner  as  a  ray  of  white  light 
can  be  modified  and  divided,  so  a  ray  of  radiant  heat  can  be  separated  into 
parts  possessing  qualities  corresponding  to  the  various  colors. 

SECTION    III. 

THE     EFFECTS     OF     HEAT. 

what    effect        519.  The  general  and  most  obvious  effect  of 


^eat  UPOD  material  substances,  is  to  expand 
bodies  ?  them,  or  increase  their  dimensions. 


228  WELLS'S  NATUKAL  PHILOSOPHY. 

is  the  form  of  520.  The  form  of  all  bodies  appears  to  be 
pendeutlebup<m  entirely  dependent  on  heat  ;  by  its  increase 
solids  are  converted  into  liquids,  and  liquids 
into  vapor  ;  by  its  diminution  vapors  are  condensed  into 
liquids,  and  these  in  turn  become  solids. 

If  matter  ceased  to  be  influenced  by  heat,  all  liquids,  vapors,  and  doubtless 
even  gases,  would  become  permanently  solid,  and  all  motion  on  the  surface  of 
the  earth  would  be  arrested. 

what  are  the        521.  The  three  most  apparent  effects  of 
parentm(Sea£    neat?  so  far  as  relate    to  the  form  and  dimen- 
sions of  bodies,  are  Expansion,  Liquefaction, 
and  Vaporization. 

Heat  operates  to  produce  expansion  by  introducing  a  repulsive  force  among 
the  particles  of  the  body  it  pervades.  This  repulsive  force,  in  the  case  of 
solids,  weakens  or  overcomes  the  attraction  of  cohesion,  and  gives  to  the  par- 
ticles of  all  matter  a  tendency  to  separate,  or  increase  their  distance  from  one 
another.  Hence  the  general  mass  of  the  body  is  made  to  occupy  a  larger 
space,  or  expand.  ' 

in  what  bodies  522.  The  expansion  occasioned  by  heat  is 
dncethegrS-  greatest  in  those  bodies  which  are  the  least  in- 
est  expansion?  fluenced  by  the  attraction  of  cohesion.  Thus 
the  expansion  of  solids  is  comparatively  trifling,  that 
of  liquids  much  greater,  and  that  of  gases  very  consid- 
erable. 

DO  bodies  con-  523.  The  expansion  of  the  same  body  will 
S  al0  long  continue  to  increase  with  the  quantity  of  heat 


them6?*  enters     tnat  enters  i*>  so  l°no  as  tne  f°rm  and  chemi- 

cal constitution  of  the  body  is  preserved. 
524.  Among  solids  the  metals  expand  the  most  ;  but 
an  iron  wire  increases  only  1-282  in  bulk  when  heated  from 
32°  of  the  thermometer  up  to  212. 

Solids  appear  to  expand  uniformly  from  the  freezing  point  of  water  up  to 
212°,  the  boiling  point  of  water  ;  —  that  is  to  say,  the  increase  of  volume  which 
attends  each  degree  of  temperature  which  the  body  receives  is  equal.  When 
solids  are  elevated,  however,  to  temperatures  above  212°,  they  do  not  dilate 
uniformly,  but  expand  in  an  increasing  ratio. 

The  expansion  of  solids  by  heat  is  clearly  shown  by  the  following  experi- 
ment Fig.  202  :  m  represents  a  ring  of  metal,  through  which,  at  the  ordinary 
temperature,  a  small  iron  or  copper  ball,  a,  will  pass  freely,  this  ball  being  a 
little  less  than  the  diameter  of  the  ring.  If  this  ball  be  now  heated  by  the 


THE  EFFECTS  OF   HEAT. 


:229 


What  applica- 
tions of  the 
expansion  of 
solids  by  heat 
are  made  in 
the  arts? 


flame  of  an  alcohol  lamp,  it  will  become  so  .         FIG.'  202. 

far  expanded  by  heat  as  no  longer*  to  pass 
through  the  ring. 

The  expansion  of  solids 

by  heat  is  made  applicable 

for  many  useful  purposes  in 

the  arts.  The  tires  of  wheels, 

and  hoops  surrounding 
water-vats,  barrels,  etc.,  are  made  in  the 
first  instance  somewhat  smaller  than  the 
frame- work  they  are  intended  to  surround. 
They  are  then  heated  red  hot  and  put  on 
in  an  expanded  condition ;  on  cooling,  they 
contract  and  bind  together  the  several  parts  with  a  greater  force:  than  coultl 
be  conveniently  applied  by  any  mechanical  means.  In  like  manner,  in  con- 
structing steam-boilers,  the  rivets  are  fastened  while  hot,  in  order  that  they 
may,  by  subsequent  contraction,  fasten  the  plates  together  more  firmly.  - 

M      525.  The  force  with  which  bodies  expand 

With  what  de-K      ,  i          ,  t         •     /i  /•    .1         • 

gree  of  force    and  contract  under  the  influence  of  the  m- 
Pand°and8coJ-     crease  or  diminution  of  heat,  is  apparently 
irresistible,   and  is  recognized  as  one  of  the 
greatest  forces  in  nature. 

The  amount  of  force  with  which  a  solid  body  will  expand  or  contract  is 
equal  to  that  which  would  be  required  to  compress  it  through  a  space  equal 
to  its  expansion,  and  to  that  which  would  be  required  to  stretch  it  through  a 
space  equal  to  its  contraction.  Thus,  if  a  pillar  of  metal  one  hundred  inches 
in  height,  being  raised  in  temperature,  is  augmented  in  height  by  a  quarter 
of  an  inch,  the  force  with  which  such  increase  of  height  is  produced  is  equal 
to  a  weight  which  being  placed  upon  the  top  of  the  pillar  would  compress  it 
BO  as  to  diminish  its  height  by  a  quarter  of  an  inch. 

In  the  same  manner,  if  a  rod  of  metal,  one  hundred  inches  in  length,  be 
contracted  by  diminished  temperature,  so  as  to  render  its  length  a  quarter  of 
an  inch  less,  the  force  with  which  this  contraction  takes  place  is  equal  to  that 
which  being  applied  to  stretch  it  would  cause  its  length  to  be  increased  by  a 
quarter  of  an  inch. 

This  principle  is  sometimes  practically  applied  when  great  mechanical  force 
is  required  to  be  exerted  through  small  spaces.  Thus  walls  of  buildings 
which,  from  a  subsidence  of  the  foundation,  or  an  unequal  pressure,  have  been 
thrown  out  of  their  perpendicular  position,  and  are  in  danger  of  falling,  may 
be  restored  in  the  following  manner:  A  series  of  iron  rods  are  carried  across 
the  building,  passing  through  holes  in  the  walls,  and  secured  by  nuts  on  the 
outside.  The  rods  are  then  heated  by  lamps  until  they  expand,  thereby 
causing  their  ends  to  project  beyond  the  building.  The  nuts  with  which 
these  extremities  are  provided  are  then  screwed  up  until  they  are  in  close 
contact  with  the  outside  wall,  the  lamps  are  'then  withdrawn  and  the  rods 


230  WELLS'S  NATURAL   PHILOSOPHY. 

allowed  to  cool.  In  cooling  they  gradually  contract,  and  by  their  contrac- 
tion draw  up  the  walls. 

On  account  of  the  expansion  of  metal  by  heat,  the  successive  rails  which 
compose  a  line  of  railway  can  not  be  placed  end  to  end,  but  a  small  space  is 
left  between  their  extremities  for  expansion. 

A  stove  snaps  and  crackles  when  a  fire  is  first  kindled  in  it,  and  also  when 
the  fire  in  it  is  extinguished.  This  noise  is  occasioned  by  the  expansion  and 
contraction  of  the  several  parts  consequent  on  the  increase  and  diminution  of 
heat 

A  glass  or  earthen  vessel  is  liable  to  break  when  hot  water  is  poured 
into  it,  on  account  of  the  unequal  expansion  of  the  inner  and  outer  surfaces. 
Glass  and  earthen  ware  being  poor  conductors  of  heat,  the  inner  surfaces 
in  contact  with  the  hot  water  become  heated  and  expand  before  the  outer  are 
affected;  the  tendency  of  this  is  to  warp  or  bend  the  aides  unequally,  and  as 
the  brittle  material  can  not  bend,  it  breaks. 

Nails  in  old  houses  are  often  loose  and  easily  drawn  out ;  the  iron  expands 
in  summer  and  contracts  in  winter  more  than  the  wood  into  which  it  has 
been  driven,  and  thus  in  time  the  opening  is  enlarged. 

When  the  stopper  of  a  decanter  or  smelling-bottle  sticks,  a  cloth  dipped  in 
hot  water,  and  applied  to  the  neck  of  the  bottle  will  frequently  loosen  it,  since 
by  the  heat  of  the  cloth  its  dimensions  are  expanded  and  enlarged. 

The  tone  of  a  piano  is  higher  in  a  cold  than  in  a  warm  room,  for  the  reason 
that  the  strings,  being  contracted  by  cold,  are  drawn  tighter. 

526.  Liquids  expand  through  the  agency  of 

To  what  extent  \  £         J 

do  liquids  ex-    heat  more  unequally,  and  to  a  much  greater 

pandbyheat?  vj 

degree  than  solids. 

A  column  of  water  contained  in  a  cylindrical  glass 
vessel  will  expand  aV  in  length  on  being  heated  from 
the  freezing  to  the  boiling  point,  while  a  column  of 
iron,  with  the  same  increase  of  temperature,  will  expand 
only  -s^f. 

A  familiar  illustration  of  the  expansion  of  water  by  heat  is  seen  in  the  over- 
flow of  full  vessels  before  boiling  commences.  Different  liquids  expand  very 
unequally  with  an  equal  increase  in  temperature.  Spirits  of  wine,  on  being 
heated  from  32°to212°,  increase  one  ninth  in  bulk;  oil  expands  about  one 
twelfth ;  water,  as  before  stated,  about  one  twenty-third.  A  person  buying 
oil,  molasses  and  spirits  in  winter,  will  obtain  a  greater  weight  of  the  same 
material  in  the  same  measure  than  in  summer.  Twenty  gallons  of  alcohol 
bought  in  January,  will,  with  the  ordinary  increase  of  temperature,  become, 
by  expansion,  twenty -one  gallons  in  July. 

What  pecuii-  527.  Water,  as  it  decreases  in  temperature 
£»nsion°f  does  toward  the  freezing  point,  exhibits  phenomena 
water  exhibit?  wfcj:Cn  are  wholly  at  variance  with  the  general 


THE   EFFECTS   OF   HEAT.  ,231 

law  that  bodies  expand  by  beat  and  contract  by  cold,  or 
by  a  withdrawal  of  heat.* 

As  the  temperature  of  water  is  lowered,  it  continues  to  contract  until  it 
arrives  at  a  temperature  of  39°  F.,  when  all  further  contraction  ceases.  The 
volume  or  bulk  is  observed  to  remain  stationary  for  a  time,  but  on  lowering 
the  temperature  still  more,  instead  of  contraction,  expansion  is  produced,  and 
this  expansion  continues  at  an  increasing  rate  until  the  water  is  congealed. 
At  the  moment  also  of  its  conversion  into  ice,  it  undergoes  a  still  further 
expansion. 

528.  Water  attains  its  greatest  density,  or 

When  is  water  .  .        ,     .  . 

«f  the  greatest    the  greatest  quantity  is  contained  in  a  given 
balk,  at  a  temperature  of  39°  F. 

As  the  temperature  of  water  continues  to  decrease  below  39°,  the  point  of 
its  greatest  density,  its  particles,  from  their  expansion,  necessarily  occupy  a 
larger  space  than  those  which  possess  a  temperature  somewhat  more  elevated. 
The  coldest  water,  therefore,  being  lighter,  rises  and  floats  upon  the  surface 
of  the  warmer  water.  On  the  approach  of  winter  this  phenomenon  actually 
takes  place  in  our  lakes,  ponds  and  rivers.  When  the  surface-water  becomes 
sufficiently  chilled  to  assume  the  form  of  ice,  it  becomes  still  lighter,  and  con- 
tinues to  float.  By  this  arrangement,  water  and  ice  being  almost  perfect 
non-conductors  of  heat,  the  great  mass  of  the  water  is  protected  from  the 
influence  of  cold,  and  prevented  from  becoming  chilled  throughout. 

If  water  constantly  grew  heavier  as  its  temperature  diminished  (as  is  the 
case  with  most  liquids),  the  colder  particles  at  the  surface  would  constantly 
sink,  until  the  whole  body  of  water  was  reduced  to  the  freezing  point.  Again, 
if  ice  was  not  lighter  than  water,  it  would  sink  to  the  bottom,  and  by  the 
continuance  of  this  operation,  a  river  or  lake  would  soon  become  an  immense 
solid  mass  of  ice,  which  the  heat  of  summer  would  be  insufficient  to  dissolve. 
The  temperate  regions  of  the  earth  would  thus  be  rendered  uninhabitable. 
Among  all  the  phenomena  of  the  natural  world,  there  is  no  more  striking 
illustration  of  the  wisdom  of  the  Creator,  and  of  the  evidences  of  design,  than 
in  this  wonderful  exception  to  a  great  general  law. 

wh    d  The  expansion  of  water  at  the  moment  of  freezing  is  attrib- 

ter  expand  in  uted  to  a  new  and  peculiar  arrangement  of  its  particles.  Ice 
is,  in  reality,  crystallized  water,  and  during  its  formation  the 
particles  arrange  themselves  in  ranks  and  lines  which  cross  each  other  at 
angles  of  60°  and  120°,  and  consequently  occupy  more  space  than  when 
liquid.  This  may  be  seen  by  examining  the  surface  of  water  in  a  saucer  while 
freezing. 

A  beautiful  illustration  of  this  crystallization  of  water  in  freezing  is  seen  in 
the  frost-work  upon  windows  in  winter,  caused  by  the  congelation  of  the 
vapor  of  the  room  when  it  comes  in  contact  with  the  cold  surface  of  the  glass. 

*  A  few  other  liquids  besides  water  expand  with  a  reduction  of  temperature.  Fused 
iron,  antimony,  zinc,  and  bismuth,  are  examples  of  such  expansion.  Mercury  is  a  re- 
markable instance  of  the  reverse,  for  when  it  freezes,  it  suffers  a  very  great  contraction. 


232  WELLS'S  NATURAL  PHILOSOPHY. 

All  these  frost-work  figures  are  limited  by  the  laws  of  crystallization,  and  the 
lines  which  bound  them,  form  among  themselves  no  angles  but  those  of 
30°,  60°,  and  120°.  If  we  fracture  thin  ice,  by  allowing  a  pole  or  weight  to 
fall  upon  it,  the  fracture  will  have  more  or  less  of  regularity,  being  generally 
in  the  form  of  a  star,  with  six  equi-distant  radii,  or  angles  of  60°, 

529.  The  force  exerted  by  the  expansion  of  water  in  the 
force  does^a-      act  of  freezing  is  very  great.     As  an  illustration,  the  following 
ter  expand  in      experiment  may  be  quoted  :  —  Cast-iron  bomb-shells,  thirteen 

inches  in  diameter  and  two  inches  thick,  were  filled  with  wa- 
ter, and  their  apertures  or  fuse-holes  firmly  plugged  with  iron  bolts.  Thus 
prepared,  they  were  exposed  to  the  severe  cold  of  a  Canadian  winter, 
at  a  temperature  of  about  19°  below  zero.  At  the  moment  the  water 
froze,  the  iron  plugs  were  violently  thrust  out,  and  the  ice  protruded,  and 
in  some  instances  the  shells  burst  asunder,  thus  demonstrating  the  enor- 
mous interior  pressure  to  which  they  were  subjected  by  water  assuming  a 
solid  state. 

The  rounded  and  weather-worn  appearance  of  rocks  is  mainly  due  to  the 
expansion  of  freezing  water,  which  penetrates  into  their  fissures,  and  is  ab- 
sorbed into  their  pores  by  capillary  attraction.  In  freezing,  it  expands  and 
detaches  successive  fragments,  so  that  the  original  sharp  and  abrupt  outline  ia 
gradually  rounded  and  softened  down. 

The  bursting  of  earthen  water  vessels,  and  of  water  pipes,  by  the  freezing 
of  water  contained  in  them,  are  familiar  illustrations  of  the  same  principle. 

By  allowing  the  water  to  run  in  a  service-pipe,  we  prevent  its  freezing,  be- 
cause the  motion  of  the  current  prevents  the  crystals  from  forming  and 
attaching  themselves  to  the  sides  of  the  pipe. 

530.  The  ordinary  temperature  at  which  water  freezes  ia 
perature  does      32°,   Fahrenheit's   thermometer.     This  rule  applies  only  to 
water  freeze?       fresh  water;  salt  water  never  freezes  until  the  surface  is  cooled 
down  to  27°,  or  five  degrees  lower  than  the  freezing  point  of  winter. 

Under  some  circumstances  pure  water  may  be  cooled  down  to  a  tempera- 
ture much  below  32°  without  freezing.  Thus,  if  pure,  recently-boiled  water, 
be  cooled  very  slowly  and  kept  very  tranquil,  its  temperature  may  be  low- 
ered to  21°  without  the  formation  of  ice;  but  the  least  motion  causes  it  to 
congeal  suddenly,  and  its  temperature  rises  to  32°. 

Wh  in  the  531.  The  ice  produced  by  the  freezing  of  sea  or  salt  water 
ice  7prdduced  is  generally  fresh  and  free  from  salt,  since  water  in  freezing, 
&  oAalt  wa-  if  sufficient  freedom  of  motion  be  allowed  to  its  particles,  ex- 
ter  free  from  pels  all  impurities  and  coloring  matters.  The  ice  formed  in 
8alfc'  the  congelation  of  a  solution  of  indigo  is  colorless,  since  the 

water  in  which  the  indigo  was  dissolved  expels  the  blue  coloring  matter  in 
freezing. 

Blocks  of  ice  are  generally  filled  with  minute  air-bubbles  ; 
if  thl     this  is  owing  to  the  fact  that  the  water  in  freezing  expels  tho 


minute  bubbles     aj[r  contained  in  it,  and  many  of  the  liberated  bubbles  become 
lodged  and  imbedded  in  the  thickening  fluid. 


THE   EFFECTS   OF   HEAT.  233 

in  what  man-  532.  Gases  and  aeriform  substances  expand 
1  -490th  of  the  bulk  which  they  possess  at  32° 
for  every  degree  of  heat  which  they  receive 

above  that  point,  and  contract  in  the  same  proportion  for 

every  degree  of  heat  withdrawn  from  them. 

Thus,  490  cubic  inches  of  air  at  32°  would  so  expand  as  to  occupy  an  inch 
more  space  at  33°,  and  by  the  addition  of  another  degree  of  heat,  raising  its 
temperature  to  34°,  it  would  occupy  an  additional  inch,  and  so  on.  In  a  like 
manner,  by  the  withdrawal  of  heat,  490  cubic  inches  of  air  would  occupy  an 
inch  less  space  at  31°  than  at  32°  ;  two  inches  less  at  30°,  and  so  on.  The 
same  law  holds  good  for  all  other  gases,  and  for  vapors  and  steam. 
.  Illustrations  of  the  expansion  of  air  by  heat  are  most  familiar.  If  a  bladder 
partially  filled  with  confined  air  be  laid  before  the  fire,  the  air  contained  in  it 
may  be  expanded  to  a  degree  sufficient  to  burst  the  bladder.  Chestnuts  laid 
upon  a  heated  surface,  burst  with  a  loud  report  on  account  of  the  expansion 
of  the  air  within  their  shells.  The  process  of  warming  and  ventilating  build- 
ings depends  entirely  upon  the  application  of  this  principle  of  the  expansion 
and  contraction  of  air  by  the  increase  and  diminution  of  heat. 

now  may  the  533.  As  the  magnitude  of  every  body  changes 
coKcHon^of  with  the  heat  to  which  it  is  exposed,  and  as 
piled8  toe  Se  the  same  b°dy,  when  subjected  to  calorific  in- 
rfhS!t?mcllt  fluences  under  the  same  circumstances  has  al- 
ways the  same  magnitude,  the  expansions  and 
contractions  which  are  the  constant  effects  of  heat,  may  be 
taken  as  the  measure  of  the  cause  which  produced  them. 
what  are  the  534.  The  instruments  for  measuring  heat 
are  Thermometers  and  Pyrometers.  The  for- 
mer  are  llse(j  for  measuring  moderate  tempera- 
tures ;  the  latter  for  determining  the  more  elevated  de- 
grees of  heat. 

Liquids  are  better  adapted  than  either  solids  or  gases  for  measuring  the 
effects  of  heat  by  expansion  and  contraction  ;  since  in  solids  the  direct  ex- 
pansion by  heat  is  so  small  as  to  be  seen  and  recognized  with  difficulty,  and 
in  air  or  gases  it  is  too  extensive,  and  too  liable  to  be  affected  by  variations 
in  the  atmospheric  pressure.  From  both  of  these  disadvantages  liquids  are 
free. 

The  liquid  generally  used  in  the  construction  of  thermometers  is  mercury, 
or  quicksilver. 

Mercury  possesses  greater  advantages  for  this  purpose  than 
any  other  liquid.      It  is,   in  the  first  place,  eminently  dis- 


adapted  for  the     tinguished  for  its   fluidity  at   all   ordinary  temperatures;    it 
thermometers?    is,  in  addition,  the  only  body  in  a  liquid  state  whose  va- 


234  WELLS'S  NATURAL   PHILOSOPHY. 

riations  In  volume,  or  magnitude,  through  a  considerable  range  of  tempe- 
rature are  exactly  uniform  and  proportional  with  every  increase  and  dim- 
inution of  heat.  Mercury,  moreover,  boils  at  a  higher  temperature  than 
any  other  liquid,  except  certain  oils ;  and,  on  the  other  hand,  it  freezes  at 
a  lower  temperature  than  all  other  liquids,  except  some  of  the  most  vola- 
tile, such  as  ether  and  alcohol.  Thus  a  mercurial  thermometer  will  have  a 
wider  range  than  any  other  liquid  thermometer.  It  is  also  attended  with 
this  convenience,  that  the  extent  of  temperature  included  between  melting 
ice  and  boiling  water  stands  at  a  considerable  distance  from  the  limits  of  its 
range,  or  its  freezing  and  boiling  points. 

Describe  the  535.  The  mercurial  thermometer  consists  es- 
Sometre?lther"  sentially  of  a  glass  tube  with  a  bulb  at  one 
end,  partially  filled  with  mercury.  The  mer- 
cury introduced  through  an  opening  in  the  end  of  the 
tube  is  afterward  boiled,  so  as  to  expel  all  air  and  moist- 
ure, and  fill  the  tube  with  its  own  vapor.  The  open  end 
of  the  tube  is  then  closed,  by  fusing  the  glass,  and  as  the 
mercury  cools  it  contracts,  and  collects  in  the  bulb  and 
lower  part  of  the  tube,  leaving  a  vacuum  above,  through 
which  it  may  again  expand  and  rise  on  the  application  of 
heat.  In  this  condition  the  thermometer  is  complete, 
with  the  exception  of  graduation. 

2\    536.  As  thermometers  are  constructed  of  different  dimen- 
How  are  ther- 
mometers gra-      sions  and  capacities,  it  is  necessary  to  have  some  fixed  rules 

duated  ?  for  graduating  them,  in  order  that  they  may  always  indicate 

the  same  temperature  under  the  same  circumstances,  as  the  freezing-point,  for 
example.  To  accomplish  this  end  the  following  plan  has  been  adopted : — 
The  thermometers  are  first  immersed  in  melting  snow  or  ice.  The  mercury 
will  be  observed  to  stop  in  each  thermometer-tube  at  a  certain  height ;  these 
heights  are-then  marked  upon  the  tubes.  Now  it  has  been  ascertained  that 
at  whatever  time  and  place  the  instruments  may  be  afterward  immersed  in 
melting  snow  or  ice,  the  mercury  contained  in  them  will  always  fix  itself  at 
the  point  thus  marked.  This  point  is  called  the  freezing  point  of  water. 

Another  fixed  point  is  determined  by  immersing  the  instruments  in  boiling 
water.  It  has  been  found  that  at  whatever  time  or  place  the  instruments 
are  immersed  in  pure  water,  when  boiling,  provided  the  barometer  stands  at 
the  height  of  thirty  inches,  the  mercury  will  always  rise  in  each  to  a  certain 
height.  This,  therefore,  forms  another  fixed  point  on  the  scale,  and  is  called 
the  boiling  point. 

Thus  far  all  thermometers  are  constructed  alike.  In  the 
theTmometere  thermometer  most  generally  used,  and  which  is  known  as 
of  Fahrenheit  Fahrenheit's,  the  intervals  on  the  scale,  between  the  freezing 
graduated?  and  j^^g  ^j^  are  divided  into  180  equal  parts.  This 


THE    EFFECTS   OF    HEAT. 


235 


What    other 


What  consti- 
tutes the  dif- 
ference be- 
tween the  dif- 
ferent varieties 
of  the  ther- 
mometer ? 


division  is  similarly  continued  below  the  freezing  point  to        FiG.    203. 

the  place  0,  called  zero,  and  each  division  upward  from  that 

is  marked  with  the  successive  numbers  1,   2,   3,   etc.     The 

freezing  point  will  now  be  the  32d  division,  and  the  boiling 

point  will  be  the  212th  division.     These  divisions  are  called 

degrees,  and  the  boiling  point  will  therefore  be  212°,  and  the 

freezing  temperature,  32°.    Fig.  203  represents  the  usual  form 

of  thermometer,  with  its  graduated  scale. 

Thermometers  of  this  character  are  called  Fahrenheit's, 
from  a  Dutch  philosophical  instrument-maker  who  first  intro- 
duced this  method  of  graduation  in  the  year  1724. 

537.  In  addition  to  Fahrenheit's 
thermometer,  two  others  are  ex- 
tensively  used,  which  are  known 
as  Reaumur's,  and  the  Centigrade  thermom- 
eter, or  thermometer  of  Celsius. 

The  only  difference  between  these  three 
kinds  of  thermometers  is  the  difference  in 
graduating  the  interval  between  the  freezing 
and  boiling  points  of  water.  Reaumur's  is  di- 
vided into  eighty  degrees,  the  Centigrade  into 
one  hundred,  and  Fahrenheit's  into  one  hundred  and  eighty. 
According  to  Reaumur,  water  freezes  at  0°,  and  boils  at  80° ; 
according  to  Centigrade,  it  freezes  at  0°,  and  boils  at  100° ; 
and  according  to  Fahrenheit,  it  freezes  at  32°,  and  boils  at 
212°;  the  last,  very  singularly,  commences  counting,  not 
at  the  freezing  point,  but  32°  below  it. 

The  difference  between  these 
instruments  can  be  easily  seen 
by  reference  to  Fig.  204. 

In  England,  Holland,  and  the 
United  States,  the  thermometer 
most  generally  used  is  Fah- 
renheit's. Reaumur's  scale  is  used  in  Ger- 
many, and  the  Centigrade  in  France,  Sweden, 
and  some  other  parts  of  Europe.  The  scale 
of  the  Centigrade  is  by  far  the  simplest  and 
most  rational  method  of  graduation,  and  at  the 
present  it  is  almost  universally  adopted  for 
scientific  purposes. 

538.  The  thermometer  was  invented  about 
the  year  1600;  but,  like  many  other  inven- 
tions, the  merit  of  its  discovery  is  not  to  be 
ascribed  to  one  person,  but  to  be  distributed 
among  many. 


236 


WELLS'S   NATURAL   PHILOSOPHY. 


How  is  cold  of         539>  •A-S  the  temperature  is  lowered,  the  mercury  in  Fah- 
preat  intensity      renheit's  thermometer  gradually  sinks,  until  it  reaches  a  point 

39°  below  zero,  where  it  freezes.  Mercury,  therefore,  can  not 
be  made  available  for  measuring  cold  of  a  greater  intensity.  This  difficulty 
is,  however,  obviated  by  using  a  thermometer  filled  with  alcohol  colored  red, 
as  this  fluid,  when  pure,  never  freezes,  but  will  continue  to  sink  lower  and 
lower  in  the  tube  as  the  cold  increases.  Such  a  thermometer  is  called  a 
spirit  thermometer. 

How  is  heat  of         540-  Ifa  Fahrenheit's  thermometer  be  heated,  the  mercury 
great  intensity      contained  in  it  will  rise  in  the  tube  until  it  reaches  660°,  at 

which  temperature  it  begins  to  boil.  A  slight  additional  heat 
forms  vapor  sufficient  to  burst  the  tube.  Mercury,  therefore,  can  not  be  used 
to  measure  degrees  of  heat  of  greater  intensity  than  660°  F.  Temperatures 
greater  than  this  are  determined  by  means  of  the  expansion  of  solids ;  and 
instruments  founded  upon  this  principle  are  commonly  called  pyrometers. 


FIG.  205. 


FIG.  20G. 


Explain  the  The  construction  of  tne  pyrometer  is  represented  in  Fig. 
construction  of  205.  A  represents  a  metallic  bar,  fixed  at  one  end,  B,  but 
the  pyrometer.  left  free  at  the  othe^  and  m  ^^^^  with  the  end  of  a  pointer 

K,  moving  freely  over  a  graduated  scale.  If  the  bar  be  heated  by  the  flame 
of  alcohol,  the  metal  expands,  and  pressing  upon  the  end  of  the  pointer,  moves 
it,  in  a  greater  or  leas  degree.  In  this  manner,  the  effect  of  heat,  applied  for 
a  given  length  of  time,  to  bars  of  different  metals,  having  the  same  length  and 
diameter,  may  be  determined. 
w.  .  .  541.  The  first  thermometer 

>Y  nn.t      is     fin 

air-thermome-  used  consisted  of  a  column  of 
air  confined  in  a  glass  tube  over 
colored  water.  Heat  expands  the  air  and  in- 
creases the  length  of  the  column  downward, 
pushing  the  water  before  it :  cold  produces  a 
contrary  effect.  The  temperature  is  thus  indi- 
cated by  the  height  at  which  the  water  is  ele- 
vated in  the  tube.  Fig.  206  represents  the  prin- 
ciple of  the  construction  of  the  air-thermometer. 


THE   EFFECTS   OF   HEAT.  237 

A  thermometer  does  not  inform  us  how  much  heat  any  sub- 
mometer  in-  stance  contains,  but  it  merely  points  out  the  difference  in  the 
form  us  how  temperature  of  two  or  more  substances.  All  we  learn  by  the 
Sstance  con-  thermometer  is  whether  the  temperature  of  one  body  is  greater 
tains  ?  or  less  than  that  of  another ;  and  if  there  is  a  difference,  it  is 

expressed  numerically — namely,  by  the  degrees  of  the  thermometer.  It  must 
be  remembered  that  these  degrees  are  part  of  an  arbitrary  scale,  selected  for 
convenience,  without  any  reference  whatever  to  the  actual  quantity  of  heat 
present  in  bodies. 

After  the  ex-  542.  The  first  effect  produced  by  heat  upon 
SidTby  heat,  solids  is  expansion.  If  the  heat  be  augmented, 
Stal°nextoebf-  ^ey  change  their  aggregate  state  and  melt, 
served?  or  Become  liquid.  Many  solids  become  soft 

before  melting,  so  that  they  may  be  kneaded  ;  for  instance, 
wax,  glass,  and  iron.  In  this  position,  glass  can  be  bent 
and  molded  with  facility,  and  iron  can  be  forged. or  welded. 
whatisLique-  543.  %  Liquefaction  we  understand  the 
conversion  of  a  solid  into  a  liquid  by  the 
agency  of  heat,  as  solid  ice  is  converted  into  water  by  the 
heat  of  the  sun. 

Heat  is  supposed  to  convert  a  solid  into  a  liquid,  by  forcing  its  constituent 
particles  asunder  to  such  an  extent  that  the  force  of  cohesion  is  overcome  or 
destroyed. 

what  is  soiu-  544.  When  a  solid  is  immersed  in  a  liquid, 
and  gradually  disappears  in  it,  the  process 
is  termed  solution,  and  not  liquefaction.  A  solution  is 
the  result  of  an  attraction  or  affinity  between  a  solid  and 
a  fluid  ;  and  when  a  solid  disappears  in  a  liquid,  if  the 
compound  exhibits  perfect  transparency,  we  have  an  ex- 
ample of  a  perfect  solution. 

w,     .  When  a  fluid  has  dissolved  as  much  of  a  solid  as  it  is 

tion  said  to  be     capable  of  doing,  it  is  said  to  be  saturated ;  or,  in  other  words, 
the  affinity  or  attraction  of  the  fluid  for  the  solid  continues  to 
operate  to  a  certain  point,  where  it  is  overbalanced  by  the  cohesion  of  the 
solid;  it  then  ceases,  and  the  fluid  is  said  to  be  saturated. 

,T        ,  A  solution  is  a  complete  union :  a  mixture  is  a  mere  me- 

How    does    a  . 

solution  differ      chamcal  union  of  bodies. 

tur™?  a    miX"         In  most  cases>  tne  addition  of  heat  to  a  liquid  greatly  in- 
creases its  solvent  properties.     Hot  water  will  dissolve  much 
more  sugar  than  cold  water ;    and  hot  water  will  also  dissolve  many  things 
which  cold  water  is  unable  to  affect. 


238  WELLS'S  NATUKAL   PHILOSOPHY. 

what  is  va-        545.  If  heat  be  imparted  in  sufficient  quan- 

porization? 


a  state  of  vapor.  Thus,  water  being  heated  sufficiently 
will  pass  into  the  form  of  steam.  This  change  is  called 
VAPORIZATION. 

what  is  con-  ^46.  If  a  body  in  a  state  of  vapor  lose  heat 
densation?  jn  sufficient  quantity,  it  will  pass  into  a  liquid 
state.  Thus,  if  a  certain  quantity  of  heat  be  abstracted 
from  steam,  it  will  become  water.  This  change  is  called 
CONDENSATION. 

The  change  from  a  state  of  vapor  to  a  liquid  is  termed  condensation,  be- 
cause, in  so  doing,  the  body  always  undergoes  a  very  considerable  diminution 
of  volume,  and  therefore  becomes  condensed.  Most  solids  become  liquefied 
before  they  vaporize  ;  but  some  pass  at  once,  on  the  application  of  heat,  from 
the  state  of  a  solid  to  that  of  a  vapor,  without  assuming  the  liquid  condition. 

547.  The  melting  of  a  solid,  or  its  conver- 

Is  any  particu-          .••-.  -i  •        •  i  i  i  i  •>•  •, 

lar    tempera-     sion  into  a  liquid,  only  occurs  when  the  solid 

ture    requisite       .1(1  ,     •       /*        -,         •     ,        -,  ,, 

for  the  forma-    is  heated  up  to  a  certain  fixed  point  ;  but  the 

tion  of  vapors  ?  '     •     '       n        t»        •  i    •     t  A    ^  i 

conversion  01  a  liquid  into  a  vapor  takes  place 
at  all  temperatures. 

If  in  a  hot  day  we  expose  a  vessel  filled  with  cold  water  to  the  open  am 
we  find  that  the  quantity  of  water  rapidly  diminishes,  that  is,  it  evaporates, 
which  means  that  it  is  converted  into  vapor  and  diffused  through  the  air. 

what  is  the  548.  The  vapor  of  water,  and  all  other  va- 
Tapor?nce  °f  Pors>  are  invisible  and  transparent.  The  water 
which  has  become  diffused  through  the  air  by 
evaporation  only  becomes  visible  when,  on  returning  to  its 
fluid  condition,  it  forms  mist,  cloud,  dew,  or  frost. 

Steam,  which  is  the  vapor  of  boiling  water,  is  invisible,  but  when  it  comes 
in  contact  with  air.  which  is  cooler,  it  becomes  condensed  into  small  drops, 
and  is  thus  rendered  visible. 

The  proof  of  this  may  bo  found  in  examining  the  steam  as  it  issues  from 
an  orifice,  or  the  spout  of  a  boiling  kettle  :  for  a  short  space  next  to  the  open- 
ing no  steam  can  be  seen,  since  the  air  is  not  able  to  condense  it  ;  but  as  it 
spreads  and  comes  in  contact  with  a  larger  volume  of  air,  the  invisible  vapor 
becomes  condensed  into  drops,  and  is  thus  rendered  visible. 

The  visible  matter  popularly  called  steam,  should  be,  therefore,  distin- 
guished from  steam  proper,  or  the  aeriform  state  of  water.  The  cloud,  or 
smoke-like  matter  observed,  is  really  not  an  air  or  vapor  at  all,  but  a  collec- 
tion of  minute  bubbles  of  water,  wafted  by  a  current  either  of  true  steam,  or, 
more  frequently,  of  mere  moist  air. 


THE  EFFECTS  OF  HEAT. 


239 


IB     a    boiling 
temperature 
requisite      for 
the  production 


The  myriads  of  minute  globules  of  water  into  which  the  steam  is  condensed 
are  separately  invisible  to  the  naked  eye,  but  each,  nevertheless,  reflects  a 
minute  ray  of  white  light.  The  multitude  of  these  reflecting  points,  there- 
fore, make  the  space  through  which  they  are  diffused  appear  like  a  cloudy 
bodr,  more  or  less  white,  according  to  their  abundance. 

The  surface  of  any  watery  liquid,  whose  temperature  is 
about  20°  warmer  than  any  superincumbent  air,  rapidly  gives 
off  true  steam.  It  is  not  necessary,  therefore,  for  the  produc- 
tion of  steam  that  water  should  be  raised  to  the  boiling  tem- 
perature. 

IB  vapor  ai-        549.  Air  without  vapor  (theoretically  called 
™a?r?preseat     dry  air)  is  not  known  to  exist  in  nature,  and  is 
probably  not  producible  by  art. 

550.  Liquids  in  passing  into  vapors  occupy 
a  mucb  greater  space  than  the  substances  from 
which  they  are  produced.      Water,  in  pass- 
ing from  its  point  of  greatest   density  into 
steam,  expamj^  to  nearly  1700  times  its  volume. 

Fig.   207   represents  the   comparative  I  p          ^ 

volume  of  water  and  steam. 

is  the  density  551.  Vapors  are 
?0frm ?°rs  uni"  of  a11  degrees  of 
density.  The  va- 
por of  water  may  be  as  thin  as 
air,  or  almost  as  dense  as  water. 

The  opinion  formerly  prevailed  that  va- 
pors could  not  exist  by  themselves  as 
such,  but  that  they  were  dissolved  in  the 
air  in  the  same  way  as  salt  is  dissolved  in 
water.  The  fallacy  of  this  idea  is  proved 
by  the  fact  that  evaporation  goes  on  more 
rapidly  in  a  vacuum,  where  no  substance  whatever  is  present,  than  in  the 


What  is  the 
relative  space 
occupied  by 
liquids  and  va- 
pors? 


What  circum- 
stances influ- 
ence evapora- 
tion? 


552.  Evaporation  takes  place  from  the  sur- 
faces of  bodies  only,  and  is  influenced  in   a 
great  degree  by  the  temperature,  dryness,  still- 
ness, and  density  of  the  atmosphere. 

„      .  The  effect  of  temperature  in  promoting  evaporation  may  be 

perature  influ-      illustrated  by  placing  an  equal  quantity  of  water  in  two  sau- 
tion8?  evapora"      cers,  one  of  which  is  placed  in  a  warm  and  dry,  and  the  other 
in  a  cold  and  damp,  situation.    The  former  will  be  quite  dry- 
before  the  latter  has  suffered  an  appreeiable  diminution. 


240  WELLS'S  NATURAL  PHILOSOPHY. 

How  does  the  When  water  is  covered  by  a  stratum  of  dry  air,  the  evapo- 
Btate  of  the  air  ration  is  rapid,  even  when  its  temperature  is  low ;  whereas  it 
oration?  ****"  &oes  on  verv  sl°wly  if  the  atmosphere  contains  much  vapor, 
even  though  the  air  be  very  warm. 

Evaporation  is  far  slower  in  still  air  than  in  a  current.  The  air  imme- 
diately in  contact  with  the  water  soon  becomes  moist,  and  thus  a  check  is 
put  to  evaporation.  But  if  the  air  be  removed  by  wind  from  the  surface  of 
the  water  as  soon  as  it  has  become  charged  with  vapor,  and  its  place 
supplied  with  fresh  air,  then  the  evaporation  continues  on  without  inter, 
ruption. 

Evaporation  is  by  no  means  confined  to  the  surface  of  liquids ;  but  takes 
place  from  the  surface  of  the  soil,  and  from  all  animal  and  vegetable  produc- 
tions. Evaporation  takes  place  to  a  very  considerable  extent  from  the  sur- 
face of  snow  and  ice,  even  when  the  temperature  of  the  air  is  far  below  the 
freezing  point 

What  sin  ular          ^^'  ~^"  ver^  s^nSu^ar  circumstance  is  connected  with  the 
circumstance        diffusion  of  vapors  throughout  the  atmosphere,  viz. :  that  the 

withCthendiffud-     vaP°rs   of   aU  bodies  arise  into  any  space  filled  with  air,  in 
sion  of  vapors?     the  same  manner  as  if  air  were  not  present,  the  two  fluids 
>  seeming  to  be  independent  of  each  other.  ^ 

Thus  as  much  vapor  of  water  can  be  forced  into  a  vessel  filled  with  air  as 
into  one  from  which  the  air  has  been  exhausted. 

554.  When  a  drop  of  water  falls  upon  a  surface  highly 
phenomena  of  heated,  as  of  metal,  it  will  be  observed  to  roll  along  the  Gur- 
a^'8tate»r0iof  face  witnout  Adhering,  or  immediately  passing  into  vapor, 
liquids.  The  explanation  of  this  is,  that  the  drop  of  water  does  not  in 

reality  touch  the  heated  surface,  but  is  buoyed  up  and  sup- 
ported on  a  layer  of  vapor  which  intervenes  between  the  bottom  of  the  drop 
and  the  hot  surface.  This  vapor  is  produced  by  the  heat  which  is  radiated 
from  the  hot  substance,  before  the  liquid  can  come  in  contact  with  it,  and 
being  constantly  renewed,  continues  to  support  the  drop.  The  drop  generally 
rolls  because  the  current  of  air  which  is  always  passing  over  a  heated  sur- 
face drives  it  forward.  The  drop  evaporates  slowly,  because  the  layer  of 
vapor  between  the  hot  surface  and  the  liquid  prevents  the  rapid  transmis- 
sion of  heat.  The  liquid  resting  upon  a  cushion  of  steam  continually  evolved 
from  its  lower  surface  by  heat,  assumes  a  rounded,  or  globular  shape,  as  the 
result  of  the  gravity  of  its  particles  toward  its  own  center. 

The  designation  which  has  been  given  to  the  condition  which  water  and 
other  liquids  assume  when  dropped  upon  very  hot  surfaces,  is  that  of  the 
"  spheroidal  state." 

If  the  surface  upon  which  the  liquid  rests  is  cooled  down  to  such  an  ex- 
tent that  vapor  is  not  generated  rapidly,  and  in  sufficient  quantity  to  sup- 
port the  drop,  it  will  come  in  contact  with  the  surface,  and  heat  being  conx- 
municated  by  conduction,  will  transform  it  instantly  into  steam. 

This  is  the  explanation  of  the  practice  adopted  by  laundresses  of  touching 
a  flat-iron  with  moisture  to  ascertain  whether  the  surface  is  sufficiently  hot.- 


THE    EFFECTS   OF   HEAT.  241 

If  the  temperature  of  the  iron  is  not  elevated  sufficiently,  the  moisture  wets 
the  surface,  and  is  evaporated ;  but  at  a  higher  degree  of  temperature,  tho 
moisture  is  repelled. 

The  phenomenon  of  the  spheroidal  condition  of  water  furnishes  an  explana- 
tion of  the  feats  often  performed  by  jugglers,  of  plunging  the  hands  with  im- 
punity into  molten  lead,  or  iron.  The  hand  is  moistened,  and  when  passed 
into  the  liquid  metal  the  moisture  is  vaporized,  and  interposes  between  the 
metal  and  the  skin  a  sheath  of  vapor.  In  its  conversion  into  vapor,  the 
moisture  absorbs  heat,  and  thus  still  further  protects  the  skin. 

whnt  is  ebui-  555.  When  a  liquid  is  heated  sufficiently  to 
ntion?  form  steain,  the  production  of  vapor  takes 
place  principally  at  that  part  where  the  heat  enters  ;  and 
when  the  heating  takes  place  not  from  above,  but  from 
the  bottom  and  sides,  the  steam  as  it  is  produced  rises  in 
bubbles  through  the  liquid,  and  produces  the  phenomenon 
of  boiling,  or  ebullition. 

what  is  the  556.  The  temperature  at  which  vapor  rises 
boiling  point?  w^  sugicient  freedom  to  cause  the  phenome- 
non of  ebullition,  is  called  the  boiling  point, 
is  the  boiiin"  557.  Different  liquids  boil  at  different  tem- 
iSquidfthe  peratures.  The  boiling  point .  of  a  liquid  is, 
same?  therefore,  one  of  its  distinctive  characters. 

Thus  water,  under  ordinary  circumstances,  begins  to  boil  when  it  is  heated 
up  to  212°  F. ;  alcohol  at  173°;  ether  at  96°;  syrup  at  221°;  Unseed  oil 
at  640°. 
What  is  sim-         Tlie  £entle  tremor,  or  undulation,  on  the  surface  of  water 

mering?  which  precedes  boiling,  and  which  is  termed  "  simmering,'-  is 
owing  to  the  collapse  of  the  bubbles  of  steam  as  they  shoot 
upward  and  are  condensed  by  the  colder  water.  The  first  bubbles  which 
form  are  not  steam,  but  air  which  the  heat  expels  from  the  water.  As  the 
temperature  of  the  whole  mass  of  the  water  increases,  the  bubbles  are  no 
longer  condensed  and  collapsed,  but  rise  through  to  the  surface ;  and  the 
moment  that  this  takes  place  boiling  commences.  The  singing  of  a  tea-kettle 
before  boiling  is  occasioned  by  the  irregular  escape  of  the  air  and  steam  ex- 
pelled from  the  water  through  the  spout  of  the  tea-kettle,  which  acts  in  the 
manner  of  a  wind-instrument  in  producing  a  sound. 

How  does  th  ^^'  -^u^s,  in  general,  being  boiled  in  open  vessels,  are 

pressure  of  the  subjected  to  the  pressure  of  the  atmosphere.  The  tendency 
feSthKoTiing  of  this  Pressure  is  to  prevent  and  retard  the  particles  of 
of  liquids?  water  from  expanding  to  a  sufficient  extent  to  form  steam. 

Hence  if  the  pressure  of  the  atmosphere  varies,  as  it  does  at 
different  times  and  places,  or  if  it  be  increased  or  diminished  by  artificial 
means,  the  boiling  point  of  a  liquid  will  undergo  a  corresponding  change. 

11 


242  WELLS'S   NATURAL  PHILOSOPHY. 

How  ma  the  ^^'  ^s  we  ascen<^  ^n*°  *^e  atmosphere  the  pressure  is  di- 
temperature  at  minished,  because  there  is  less  of  it  above  us ;  it  therefore 
boiishbe  ^used  foll°ws,  that  water  at  different  heights  in  the  atmosphere  will 
for  detersnin-  boil  at  different  temperatures,  and  it  has  been  found  by  ob- 
iag elevations?  servationj  tnat  an  elevation  of  550  feet  above  the  level  of  the 
sea  causes  a  difference  of  one  degree  in  its  boiling  point.  Hence  the  boiling 
point  of  water  becomes  an  indication  of  the  height  of  any  station  above  the 
sea-level,  or  in  other  words,  an  indication  of  the  atmospheric  pressure ;  and 
thus  by  means  of  a  kettle  of  boiling  water  and  a  thermometer,  the  height  of 
the  summit  of  any  mountain  may  be  ascertained  with  a  great  degree  of  ac- 
curacy. If  the  water  boils  at  211°  by  the  thermometer,  the  height  of  tho 
place  is  550  feet;  if  at  210°,  the  height  is  1100  feet,  and  so  on,  it  being  only 
necessary  to  multiply  550  by  the  number  of  degrees  on  the  thermometer 
between  the  actual  boiling  point  and  212°,  to  ascertain  the  elevation.  In  tho 
city  of  Quito,  in  South  America,  water  boils  at  194°  2"  F. ;  its  height  abovo 
the  sea-level  is,  therefore,  9,541  feet. 

As  we  descend  into  mines,  the  pressure  of  the  atmosphere  is  increased,  tlicro 
being  more  of  it  above  us  than  at  the  surface  of  the  earth.  "Water,  therefore, 
must  be  heated  to  a  higher  temperature  before  it  will  boil,  and  it  has  been 
found  that  a  descent  of  550  feet,  as  before,  makes  a  difference  of  one  degree. 
560.  In  a  like  manner,  if  by  artificial  means  we  increase  or 
boiling'  point  diminish  the  pressure  of  the  atmosphere  on  the  surface  of  a 
chan^d^arti3  %u^i  we  change  its  boiling  point.  If  water  be  heated  in  a 
ficially?  vacuum,  ebullition  will  commence  at  a  point  140°  lower  than 

in  the  open  air.  If  a  vessel  of  ether  be  placed  under  the  re- 
ceiver of  an  air-pump,  and  the  atmospheric  pressure  removed  from  its  surfac^ 
the  vapor  rises  so  abundantly  that  ebullition  is  produced  without  any  in- 
crease of  temperature. 

H       -s  Several  beautiful  applications  in  the  arts  have  been  made 

boiled  in   the      of  the  principle  that  liquids  boil  at  a  lower  temperature  when 
fiSn68?  °f  re~      freed  from  the  Pressure  of>  tne  atmosphere  than  in  the  open 
air. 

In  the  refining  of  sugar,  if  the  syrup  is  boiled  in  the  open  air,  the  tempera- 
ture of  the  boiling  point  is  so  high  that  portions  of  the  sugar  become  decom- 
posed by  the  excess  of  heat,  and  lost  or  injured ;  the  syrup  is  therefore  boiled 
in  close  vessels  from  which  the  air  has  been  previously  exhausted,  and  in  this 
way  the  water  of  the  syrup  may  be  evaporated  at  a  temperature  so  low  as  to 
prevent  all  injury  from  heat. 

For  cooking,  this  application  could  not  be  carried  out.  The  water  might, 
indeed,  be  made  to  boil  at  a  temperature  much  less  than  212°,  but  owing  to 
its  diminished  heat  would  not  produce  the  desired  effect. 

what  is  dista-        561.  Distillation  is  a  process  by  which  one 

lation?         k0jy  jfl  geparated  from  another  by  means  of 

heat,  in  cases  where  one  of  the  bodies  assumes  the  form 

of  vapor  at  a  lower  temperature  than  the  other  ;  this  first 


THE   EFFECTS   OF    HEAT. 


243 


FIG.  208. 


rises  in  the  form  of  vapor,  and  is  received  and  condensed 
in  a  separate  vessel. 

By  this  means  very  volatile  bodies  can  be  easily  separated  from  less  vola- 
tile ones ;  as  brandy  and  alcohol  from  the  less  volatile  water  which  may  be 
mixed  with  them.  Water  of  extreme  purity  can  also  be  obtained  by  distil- 
lation, because  the  non-volatile  and  earthy  substances  contained  in  all  spring 
waters  do  not  ascend  with  the  vapor,  but  remain  behind  in  the  vessel. 

Distillation  upon  a  small 
scale  is  effected  by  means 
of  a  peculiar-shaped  vessel, 
called  a  retort,  Fig.  208, 
which  is  half  filled  with  a 
volatile  liquid  and  heated ; 
the  steam,  as  it  forms, 
passes  through  the  neck  of 
the  retort  into  a  glass  re- 
ceiver set  into  a  vessel  filled 
with  cold  water,  and  is  then 
condensed. 

"When  the  operation  of  distillation  is  conducted  on  an  extensive  scale,  a  large 
FIG    209  vessel  called  a  "  stilt"  is  used,  and,  for  con- 

densing the  vapor,  vats  are  constructed, 
holding  serpentine  pipes,  or  "worms," 
which  present  a  greater  condensing  sur- 
face than  if  the  pipe  had  passed  directly 
through  the  vat.  To  keep  the  coil  of  pipe 
cool,  the  vats  are  kept  filled  with  cold 
water.  In  Fig.  209,  a  is  a  furnace,  in  which 
.is  fixed  a  copper  vessel,  or  still,  to  contain 
the  liquid.  Heat  being  applied,  the  steam 
rises  in  the  head,  6,  and  passes  through 
the  worm,  d,  which  is  placed  in  a  vessel 
of  water,  the  refrigerator.  The  vapor 
thus  generated  is  condensed  in  its  passage,  and  passes  out  as  a  liquid  by  the 
external  pipe  into  a  receiver. 

What  is  the  The  differenc6  between  drying  by  heat  and  distillation  is, 
difference  be-  that  in  one  case,  the  substance  vaporized,  being  of  no  use,  is 
fcr^heat  r3and  allowed  to  escape  or  become  dissipated  in  the  atmosphere ; 
distillation  ?  while  in  the  other,  being  the  valuable  part,  it  is  caught  and 
condensed  into  the  liquid  form.  The  vapor  arising  from  damp  linen,  if  caught 
and  condensed  would  be  distilled  water ;  the  vapor  given  out  by  bread  while 
baking,  would,  if  collected,  be  a  spirit  like  that  obtained  in  the  distillation  cf 
grain. 

What  is  snbli-          5G2'  ^a  8Ome  substances>  bv  tae  application  of  heat,  pass 

mation?  directly  from  the  solid  condition  to  the  state  of  vapor,  so  somo 

substances,  as  camphor,  sulphur,  arsenic,  etc.,  when  vaporized 


244  WELLS'S   NATURAL   PHILOSOPHY. 

by  heat,  deposit  th^ir  condensed  vapors  in  a  solid  form.     This  process  is 
termed  sublimation^ , 

what  remark-  563.  One  of  the  most  remarkable  circum- 
stances  which  accompany  the  phenomena, 
botn  of  liquefaction  and  vaporization,  is  the 
disappearance  of  the  heat  which  has  effected 

the  change. 

How  may  this  Thus,  if  a  thermometer  be  applied  to  a  mass  of  snow,  or  ico 
principle  be  il-  just  upon  the  point  of  melting,  it  will  be  found  to  stand  at 
32°  F.  If  the  ice  be  placed  in  a  vessel  over  a  fire,  and  tho 
temperature  tested  at  the  moment  it  has  entirely  melted,  the  water  produced 
will  have  only  the  temperature  of  32°,  the  same  as  that  of  the  original  ice. 
Heat,  however,  during  the  whole  process  of  melting,  has  been  passing  rapidly 
into  the  vessel  from  the  fire,  and  if  a  quantity  of  mercury,  or  a  solid  of  the 
same  size,  had  been  exposed  to  the  same  amount  of  heat,  it  would  have  con- 
stantly increased  in  temperature.  It  is  clear,  therefore,  that  the  conversion  of 
ice,  a  solid,  into  water,  a  liquid,  has  been  attended  with  a  disappearance  of  heat. 

Again :  if  one  pound  of  water,  having  a  temperature  of  174°,  be  mixed  with 
one  pound  of  snow  at  32°,  we  shall  obtain  two  pounds  of  water,  having  a 
temperature  of  32°.  All  the  heat,  therefore,  which  was  contained  in  tho 
hot  water  is  no  longer  to  be  detected  by  the  thermometer,  it  having  been  en- 
tirely used  up,  or  disposed  of  in  converting  snow  at  32°  into  water  at  32°. 
Such  disappearances  always  occur  whenever  a  solid  is  converted  into  a  liquid. 

If,  however,  a  pound  of  water  at  32°,  instead  of  ice  at  the  same  tempera- 
ture, had  been  mixed  with  a  pound  of  water  at  ]  74°,  we  shall  obtain  two 
pounds  at  103°,  a  temperature  exactly  intermediate  between  the  temperatures 
of  the  components  But  if  the  pound  at  32°  had  been  solid  instead  of  liquid, 
then  the  mixture,  as  before  explained,  would  have  had  a  temperature  of  32°. 
It  is  evident,  therefore,  that  it  is  the  process  of  liquefaction,  and  it  alone,  which 
renders  latent  or  insensible  all  that  heat  which  is  sensible  when  the  pound 
of  water  at  32°  is  liquid. 

In  the  same  manner  heat  disappears  when  a  liquid  is  con- 
absorption  of  verted  into  a  vapor.  The  absorption  of  heat,  in  this  instance, 
ration" beVren~  ma^T  be  easilJ  rendered  perceptible  to  the  feelings  by  pouring 
dered  evident?  a  few  drops  of  some  liquid  which  readily  evaporates,  such  as 
ether,  alcohol,  etc.,  upon  the  hand.  A  sensation  of  cold  is  immediately  ex- 
perienced, because  the  hand  is  deprived  of  heat,  which  is  drawn  away  to  effect 
the  evaporation  of  the  liquid.  On  the  same  principle,  inflammation  and  fever- 
ish heat  in  the  head  may  be  allayed  by  bathing  the  temples  with  Cologne 
water,  alcohol,  vinegar,  etc. 

If  we  surround  the  bulb  of  a  thermometer  loosely  with  cotton,  and  then 
moisten  the  latter  with  ether,  the  thermometer  will  speedily  fall  several  degrees. 
Wh  can  not  Water  when  placed  in  a  vessel  over  a  fire,  gradually  at- 
water  impart  tains  the  boiling  temperature,  or  212°;  but  afterward,  how- 
n  Cf  ever  much  we  may  increase  the  fire»  'lt  becomes  no  hotter,  all 


THE   EFFECTS   OF   HEAT.  245 

the  heat  which  is  added  serving  only  to  convert  the  water  at  212°  from  a 
liquid  condition  into  steam,  or  vapor,  at  212°. 

564.  If  we  immerse  a  thermometer  in  boiling  water,  it 
?n°o^  dthatWe  stands  at  212°  ;  if  we  place  it  in  steam  immediately  above  it, 
steam  at  2 12'  jt  indicates  the  same  temperature.  We  know,  however,  that 
wate?  "t  *  the  steam  contains  more  heat  than  boiling  water,  because  if  we 
same  tempera-  mix  an  ounce  of  water  at  212°  with  five  and  a  half  ounces  of 

water  at  32°,  we  obtain  six  and  a  half  ounces  of  water  at  a 
temperature  of  about  60°  ;  but  if  we  mix  an  ounce  of  steam  at  212°  with  five 
and  a  half  ounces  of  water  at  32°,  we  obtain  six  and  a  half  ounces  of  water  at 
212°.  The  steam,  from  which  the  increased  heat  is  all  derived,  contains  as 
much  more  heat  than  the  ounce  of  water  at  the  same  temperature,  as  would 
be  necessary  to  raise  six  and  a  half  ounces  of  water  from  the  temperature  of 
60°  to  212°,  or  six  and  a  half  times  as  much  heat  as  would  be  requisite  to 
raise  one  ounce  of  water  through  about  152°  of  temperature.  This  quantity 
of  heat  will,  therefore,  be  found  by  multiplying  152°  by  six  and  a  half, 
which  will  give  a  product  of  983° — the  excess  of  heat  contained  in  an 
ounce  of  steam  at  212°  over  that  contained  in  an  ounce  of  boiling  water  at 
the  same  temperature. 

What  b  c  ^^'  ^n  ^e  conversi°n  °f  solids  into  liquids,  and  liquids  into 

of  the  heat  vapors  by  heat,  we  may  suppose  the  heat,  the  solid,  and  the 
pcar^in  Uque-  ^quid  *°  have  respectively  combined  together; — forming  a 
faction  and  r»-  liquid  in  the  one  case,  and  a  vapor  in  the  other.  A  liquid, 

therefore,  may  be  regarded  as  a  compound  of  a  solid  and 
heat,  and  a  vapor  as  a  compound  of  heat  and  the  liquid  from  which  it  was 
formed.  The  heat  which  disappears  in  these  combinations  is  called  LATENT, 
or  COMPOUND  HEAT. 

What  are  ^G  a^sorpti°n  °f  nea*  consequent  on  the  conversion  of 

freezing  mix-      solids  into  li quids,  has  been  taken  advantage  of  in  the  arts  for 

the  production  of  artificial  cold ;  and  the  compounds  of  dif- 
ferent substances  which  are  made  for  this  purpose,  are  called  freezing  mix- 
tures. 

Why  does  the  The  most  simPle  freezmg  mixture  is  snow  and  salt.  Salt 
mixture  of  dissolved  in  water  would  occasion  a  reduction  of  temperature, 
p?oTucenintenlse  but  when  the  chemical  relations  of  two  solids  are  such,  that 
cold?  on  mixing,  both  are  rendered  liquid,  a  still  greater  degree  of 

cold  is  produced.  Such  a  relation  exists  between  salt  and  snow,  or  ice,  and 
therefore  the  latter  substances  are  used  in  preference  to  water.  When  the 
two  are  mixed,  the  salt  causes  the  snow  to  melt  by  reason  of  its  attraction 
for  water,  and  the  water  formed  dissolves  the  salt :  so  that  both  pass  from 
the  solid  to  the  liquid  condition.  If  the  operation  is  so  conducted  that  no 
heat  is  supplied  from  any  external  source,  it  follows  that  the  heat  absorbed 
in  liquefaction  must  be  obtained  from  the  salt  and  snow  which  comprise  the 
mixture,  and  they  must  therefore  suffer  a  depression  of  temperature  propor- 
tional to  the  heat  which  is  rendered  latent. 

In  this  way  a  degree  of  cold  equal  to  40°  below  the  freezing  point  of 


246  WELLS'S  NATURAL   PHILOSOPHY. 

How  great  a     water  may  be  obtained.     The  application  of  this  experiment 
csSTbe  obfad!£     t0  the  freezing  of  ice-creams  is  familiar  to  all. 
ed  by  freezing          By-mixing  snow  and  sulphuric  acid  together  in  proper  pro- 
mixtures?  portions,  a  temperature  of  90°  below  zero  can  be  obtained 
without  difficulty. 

Wh  is  the  air  ^ne  a*r  m  ^e  sP1^nS  °f  *ne  year,  when  the  ice  and  snow 
in  spring  cold  are  thawing,  is  always  peculiarly  cold  and  chilly.  This  is  due 
and  chilly?  to  the  constant  absorption  of  heat  from  the  air  by  the  ice  and 
snow  in  their  transition  from  a  solid  to  a  liquid  state.  '.  :^-?;r 

__  A  shower  of  rain  cools  the  air  in  summer,  because  the  earth 

showerinsum-  and  the  air  both  part  with  their  heat  to  promote  evaporation, 
mer  cool  the  jn  a  ]fce  manner?  the  sprinkling  of  a  hot  room  with  water 

cools  it. 

Why  is  the  The  draining  of  a  country  increases  its  warmth,  since  by 
warmth  of^a  withdrawing  the  water,  evaporation  is  diminished,  and  less 
moted  by  heat  is  subtracted  from  the  earth. 

draining  ?  rp^e  d^g^  arising  from  wet  feet  and  clothes  is  owing  to 

Why  do  wet  the  absorption  of  heat  from  the  body  by  the  evaporation  from 
£end°to  inf^i?  *^e  surfaces  of  the  wet  materials ;  the  temperature  of  the  body 
the  health  of  is  in  this  way  reduced  below  its  natural  standard,  and  the 
the  body?  proper  circulation  of  the  blood  interrupted. 

566.  The  absorption  of  heat  in  the  process  by  which  liquids 
tin  vesse^con*  are  converted  into  vapor,  will  explain  why  a  vessel  containing 
taioing  water  a  liquid  that  is  constantly  exposed  to  the  action  of  fire,  can 
fire  destroyed?  never  receive  such  a  degree  of  heat  as  would  destroy  it.  A 

tin  kettle  containing  water  may  be  exposed  to  the  action  of 
the  most  fierce  furnace,  and  remain  uninjured ;  but  if  it  be  exposed,  without 
containing  water,  to  the  most  moderate  fire,  it  will  soon  be  destroyed.  The 
heat  which  the  fire  imparts  to  the  kettle  containing  water  is  immediately  ab- 
sorbed by  the  steam  into  which  the  water  is  converted.  So  long  as  water  is 
contained  in  the  vessel,  this  absorption  of  heat  will  continue  ;  but  if  any  part 
of  the  vessel  not  containing  water  be  exposed  to  the  fire,  the  metal  will  be 
fused,  and  the  vessel  destroyed. 

567.  When  vapors  are  condensed  into  liq- 

Under       what         ...  -IT-T  i  t    •     ,  i- •,          i 

circumstances     uids.  and  liquids  sue  changed  into  solids,  the 

does  latent  heat     ,          ',     .  J     .        ,     .        °  .  n  > 

become  sensi-    latent  heat  contained  in  them  is  set  free,  or 
made  sensible. 

If  water  be  taken  into  an  apartment  whose  temperature  is  several  degrees 
below  the  freezing  point,  and  allowed  to  congeal,  it  will  render  the  room  sen- 
sibly warmer.  It  is,  therefore,  in  accordance  with  this  principle  that  tubs  of 
water  are  allowed  to  freeze  in  cellars  in  order  to  prevent  excessive  cold. 

It  is  from  this  cause  that  oceans,  seas,  and  other  large  collections  of  water 
are  most  powerful  agents  in  equalizing  the  temperature  of  the  inhabited  parts 
of  the  globe.  In  the  colder  regions,  every  ton  of  water  converted  into  ice 
out  and  diffuses  in  the  surrounding  region  as  much  heat  as  would 


THE   EFFECTS   OF   HEAT.  247 

raise  a  ton  of  water  from  32°  to  174° ;  and,  on  the  other  hand,  when  a  rise 
of  temperature  takes  place,  the  thawing  of  the  ice  absorbs  a  like  quantity  of 
heat:  thus,  in  the  one  case,  supplying  heat  to  the  atmosphere  when  the  tem- 
perature falls ;  and,  in  the  other,  absorbing  heat  from  it  when  the  temperature 
rises. 

In  the  winter,  the  weather  generally  moderates  on  the  fall  of  snow ;  snow 
is  frozen  water,  and  in  its  formation  heat  is  imparted  to  the  atmosphere,  and 
its  temperature  increased. 

Steam,  on  account  of  the  latent  heat  it  contains,  is  well 
WheciSlSteam  adaPted  for  tue  wanning  of  buildings,  or  for  cooking.  In 
adapted  for  passing  through  a  line  of  pipes,  or  through  meat  and  vegeta- 
cookfogf  and  bles»  lt  'ls  condensed,  and  imparts  to  the  adjoining  surfaces 
nearly  1000°  of  the  latent  heat  which  it  contained  before 
condensation. 

Steam  burns  much  more  severely  than  boiling  water,  for  the  reason  that 
the  heat  it  imparts  to  any  surface  upon  which  it  is  condensed  is  much  greater 
than  that  of  boiling  water. 

is  the  quantity  568.  All  bodies  con  tain  incorporated  witb 
bodiM*  theaU  them  more  or  less  of  heat ;  but  equal  weights 
same?  of  dissimilar  substances,  having  the  same  sen- 

sible temperature,  contain  unequal  quantities  of  heat. 

H  a  this  Thus  if  we  place  a  pound  of  water  and  a  pound  of  mercury 
be  demon-  over  a  fire,  it  will  be  found  that  the  mercury  will  attain  to  any 
strated?  given  temperature  much  quicker  than  the  water.  Or  if  we 

perform  the  converse  of  this  experiment,  and  take  two  equal  quantities  of 
mercury  and  water,  and  having  heated  them  to  the  same  degree  of  tempera- 
ture, allow  them  to  cooj  freely  in  the  air,  it  will  be  found  that  the  water  will 
require  much  more  time  to  cool  down  to  a  common  temperature  than  the 
mercury.  The  water  obviously  contains  more  heat  at  the  elevated  tempera- 
ture than  the  mercury,  and  therefore  requires  a  longer  time  to  cool. 
What  is  the  569*  Dissimilar  substances  require,  respectively,  different 
meaning  of  the  quantities  of  heat  to  raise  their  temperatures  one  degree  ;  and 
heat  ?  8pecrfic  tne  quantity  of  heat  necessary  to  produce  this  effect  upon  a 
body  is  termed  its  specific  heat.  In  like  manner,  the  weight 
which  a  body  includes  under  a  given  volume,  is  termed  its  specific  gravity. 

.    570.  A  substance  is  said  to  have  a  greater 

What  is  under-       ^»  t  , 

stood  bycapac-    or  less  capacity  for  heat,  according  as  a  greater 

ity  for  heat  ?  ..  •*•  ^  ' .  .    °  ' 

or  less  quantity  of  heat  is  required  to  produce 
a  definite  change  of  temperature,  or  an  elevation  of  tem- 
perature of  one  degree. 

How  does  the  In  general»  tne  capacity  of  bodies  for  heat  decreases  with 
capacity  for  heat  their  density.  Thus  mercury  has  a  less  capacity  for  heat  than 
water»  because  its  density  is  greater.  Air  that  is  rarefied,  or 
thin,  has  a  greater  capacity  for  heat  than  dense  air.  This 


248  WELLS'S   NATUKAL   PHILOSOPHY. 

circumstance  will  explain,  in  part,  the  reason  of  the  very  low  temperatures 
which  exist  at  great  elevations  in  the  atmosphere.  Persons  ascending  high 
mountains,  or  in  balloons,  find  that  the  cold  increases  with  the  elevation. 
The  reason  of  this  is,  that  as  the  air  expands  and  becomes  rarefied,  its  capac- 
ity for  heat  is  greatly  increased,  and  it  therefore  absorbs  its  own  sensible 
heat. 

What  is  the  ^n  a^  cluartera  °*  ^ie  S^°et  tne  temperature  of  the  air  at  a 
limit  of  per-  certain  height  is  reduced  so  low  by  its  rarefaction,  that  water 
petualsnow?  caQ  not  exig(.  ;n  ft  liquid  gtate>  Thig  ]imit>  the  hejght  Qf 

which  varies,  being  the  most  elevated  at  the  equator,  and  the  most  depressed 
at  the  poles,  is  called  the  line  of  PERPETUAL  SNOW.* 

Air  forcibly  expelled  from  the  mouth  feels  cool ;  in  this  instance  the  cold  is 
due  to  a  sudden  expansion  of  the  air,  by  which  its  capacity  for  heat  is  in- 
creased. 

The  capacity  for  heat  also  increases  with  the  temperature.  Thus  it  requires 
a  greater  amount  of  heat  to  elevate  the  temperature  of  platinum  from  212°  to 
213°,  than  from  32°  to  33°. 

Of  all  known  bodies,  water  has  the  greatest  capacity  for  heat. 

There  are  several  different  ways  by  means  of  which  the  ca- 
ca^aci™*^  *for  P^ty  °?  bodies  for  heat  may  be  determined.  One  method 
heat  in  differ-  consists  in  inclosing  equal  weights  of  different  bodies  heated 
SasrortSned?  to  the  same  temperature,  in  closed  cavities  in  a  block  of  ice, 
and  measuring  the  respective  quantities  of  water  which  they 
produce  by  melting  the  ice. 

The  same  result  may  also  be  obtained  by  what  is  called  the  method  of  mix- 
tures. Thus,  if  we  mix  1  pound  of  mercury  at  66°  with  1  pound  of  water  at 
32°,  the  common  temperature  will  be  33°.  Here  the  mercury  loses  33°  and 
the  water  gains  1° ;  that  is  to  say,  the  33°  of  the  mercury  only  elevates  the 
water  1°,  therefore  the  capacity  of  water  for  heat  is  33  times  that  of  mercury ; 
or,  if  we  call  the  capacity  or  specific  heat  of  water  1,  then  the  capacity  or 
specific  heat  of  mercury  will  be  l-33d  or  .0303. 

In  this  way  the  capacities  for  heat  of  a  great  number  of  bodies  has  been 
determined,  and  tables  constructed  in  which  they  are  recorded.  In  these 
tables  water  is  taken  as  the  unit  of  comparison. 

All  vapors  are  elastic,  like  air. 

eiusticityofva-  The  tendency  of  vapors  to  expand  is  unlim- 
ited ;  that  is  to  say,  the  smallest  quantity  of 
vapor  will  diffuse  itself  through  every  part  of  a  vacant 
space,  he  its  size  what  it  may,  exercising  a  greater  or  less 
degree  of  force  against  any  ohstacle  which  may  have  a 
tendency  to  restrain  it. 

*  The  line  of  perpetual  snow  at  the  equator  occurs  at  a  height  of  about  15,000  feet ;  at 
the  Straits  of  Magellan,  it  occurs  at  an  elevation  of  only  4,000  feet. 


THE   EFFECTS   OF   HEAT.  249 

The  force  with  which  a  va.por  expands  is  called  its  elastic 
force,  or  tension. 

The  elasticity  or  pressure  of  vapors  is  best  illustrated  in  the  case  of  steam, 
which  may  be  considered  as  the  type  of  all  vapors. 

When  a  quantity  of  pure  steam  is  confined  in  a  close  vessel, 
ierTshthe™ia"-  its  elastic  force  will  exert  on  every  part  of  the  interior  of  the 
tic  force  of  vessel  a  certain  pressure  directed  outward,  having  a  tendency 

to  burst  the  vessel. 
What    is    the         When  steam  is  generated  in  an  open  vessel  its  elastic  force 

steam  ffbrmed  must  be  e(lual  to  tne  elastic  force  or  pressure  of  the  atmos- 
in  an  open  res-  phere ;  otherwise  the  pressure  of  the  air  would  prevent  it  from 
forming  and  rising.  Steam,  therefore,  produced  from  boiling  wa- 
ter at  212°  F.,  is  capable  of  exerting  a  pressure  of  15  pounds  upon  every  square 
inch  of  surface,  or  one  ton  on  every  square  foot,  a  force  equivalent  to  the 
pressure  of  the  atmosphere. 

If  water  be  boiled  under  a  diminished  pressure,  and  there- 
liow  may  the 
elastic  force  of     fore  at  a  lower  temperature,  the  steam  which  is  produced  from 

crea™  dbor  di-  it;  wil1  have  a  Pressure  which  is  diminished  in  an  equal  de- 
minished?  gree.  If,  on  the  contrary,  the  pressure  under  which  water 

boils  be  increased,  the  boiling  temperature  of  the  water  and 
the  pressure  of  the  steam  formed  will  be  increased  in  a  like  proportion.  We 
have,  therefore,  the  following  rule  : — 

TO  what  is  the  571.  Steam  raised  from  water,  boiling  under 

steam5  f always  any  given  pressure,  has  an  elasticity  always 

equal?  equal  to  the  pressure  under  which  the  water 
boils. 

How  is  steam  Steam  of  a  high  elastic  force  can  only  be  made  in  close  ves- 
of  high  elastic  sels,  or  boilers.  The  water  in  a  steam-boiler,  in  the  first  in- 
force  generated?  sianc^  boils  at  212°,  but  the  steam  thus  generated  being 
prevented  from  escaping,  presses  on  the  surface  of  the  water  equally  as  on 
the  surface  of  the  boiler,  and  therefore  the  boiling  point  of  the  water  becomes 
higher  and  higher ;  or  in  other  words,  the  water  has  to  grow  constantly  hot- 
ter, in  order  that  the  steam  may  form.  The  steam  thus  formed  has  the  same 
temperature  as  the  water  which  produces  it. 

'  The  temperature  of  the  water  in  working  steam-boilers  is 

tent  can  water  always  much  greater  than  212°.  It  should  also  be  borne  in 
mind  that  water>  if  subjected  to  sufficient  pressure,  can  be 
heated  to  any  extent  without  boiling.  There  is  no  limit  to 
the  degree  to  which  water  may  be  heated,  provided  the  vessel  is  strong 
enough  to  confine  the  vapor ;  but  the  expansive  force  of  steam  is  so  enormous 
under  these  circumstances,  as  to  overcome  the  greatest  'resistance  which  has 
ever  been  exerted  upon  it 

If  a  boiler,  containing  water  thua  overheated  many  degrees  beyond  the 
boiling  point,  be  suddenly  opened,  and  the  steam  allowed  to  expand,  the 

11 


250  WELLS'S  NATURAL  PHILOSOPHY. 

whole  water  is  immediately  blown  out  of  the  vessel  as  a  mist  by  the  steam 
formed  at  the  same  instant  throughout  every  part  of  the  mass.  To  use  a 
common  expression,  "  the  water  flashes  into  steam." 

To  what  ex-  Steam,  like  water,  may  be  heated  to  any  extent  when  con- 
tent can  steam  fined  and  prevented  from  expanding  with  the  increase  of 
derprelSure?"  temperature;  in  some  of  the  methods  lately  introduced  for 
purifying  oils,  etc.,  the  temperature  of  the  steam,  before  its 
application,  is  required  to  be  sufficiently  elevated  to  enable  it  to  melt  lead. 

What  is  super-  572.  Steam  which  has  been  heated  in  a 
heated  steam?  geparate  state  to  a  high  degree  of  temperature 
under  pressure,  is  known  as  "Superheated  Steam."  In 
this  condition  its  mechanical  and  chemical  powers  are 
wonderfully  increased. 

In  the  manufacture  of  lard  on  an  extensive  scale  the  carcass  of  the  whole 
hog  is  exposed  to  the  action  of  steam  at  very  high  pressure,  this  acting  upon 
the  mass  of  flesh  and  bones,  breaks  up  and  reduces  the  whole  to  a  fat 
fluid  mass.  Ordinary  steam,  under  the  same  circumstances,  would  dissolve 
nothing. 

Steam  has  also  been  recently  applied  to  the  carbonization  of  wood.  For 
this  purpose  ordinary  steam  is  conducted  through  red  hot  pipes,  whereby  it 
attains  a  very  high  degree  of  temperature.  It  is  then  allowed  to  pass  into  a 
vessel  containing  wood  intended  to  be  converted  into  charcoal.  The  heated 
steam,  penetrating  into  the  pores  of  the  wood,  drives  off  the  volatile  portions, 
the  water,  the  tar,  etc.,  and  leaves  the  pure  carbon  alone  behind. 

what  ia  High-  573.  Steam  generated  by  water  boiling  at  a 
pressure  steam?  verv  fagfr  temperature,  is  known  as  High- 
pressure  Steam.  By  this  term  we  mean  steam  condensed 
not  by  withdrawal  of  heat,  but  by  pressure,  just  as  high- 
pressure  air  is  merely  condensed  air.  To  obtain  a  double, 
triple,  or  greater  pressure  of  steam,  we  must  have  twice, 
thrice,  or  more  steam  under  the  same  volume, 
what  relation  574.  The  sum  of  the  sensible  heat  of  any 
8en8ibieetwami  vapor,  and  the  latent  heat  contained  in  it,  is 

latent  heat?          always  the  Same. 

It  is  an  established  fact  that  the  heat  absorbed  by  vaporization  is  always 
less  the  higher  the  temperature  at  winch  this  vaporization  takes  place,  and 
just  in  proportion  also  as  vapor  or  steam  indicates  a  lower  temperature  by  the 
thermometer,  it  contains  more  latent  heat.  Thus,  if  water  boils  at  312°,  the 
heat  absorbed  in  vaporization  will  be  less  by  100°  than  if  it  boiled  at  212°. 
And  again,  if  water  be  boiled  under  a  diminished  pressure  at  112°,  the  heat 
absorbed  in  vaporization  will  be  100°  more  than  the  heat  absorbed  by  water 
boiled  at  212°. 


V 


THE   STEAM-ENGINE. 


SECTION     IV, 


251 


what  is  /\  575.  The  Steam-Engine  is  a  mechanical 
steam-Engine?  contiivance  by  which  coal,  wood,  or  other 
fuel,  is  rendered  capable  of  executing  any  kind  of  labor.* 

The  substance  which  furnishes  the  means  of  calling  the 
How  much  me- 
chanical   force     powers  of  coal  into  activity  is  water;  two  ounces  of  coal,  with 

b^th^combuif  a  Pr°Per  arrangement  will  evaporate  about  one  pint  of  water; 
tion  of  two  this  will  produce  216  gallons  of  steam,  which  can  exert  a 
ounces  of  coal?  mecnanicai-  force  equivalent  to  raising  a  weight  of  37  tons  to 
the  height  of  one  foot. 

It  has  been  found  by  experiment  that  the  greatest  amount 
How  does  the          .  .,  ... 

force  of  a  man      of  force  which  a  man  can  exert  when  applying  his  strength  to 

compare^  with  fae  best  advantage  through  the  help  of  machinery,  is  equal  to 
crated  by  *the  elevating  one  and  a  half  millions  of  pounds  -to  the  height  of 
°f  oue  foot'  b^  working  on  a  treadmill  continuously  for  eight 


hours.      A  well-constructed  steam-engine  will  perform  the 
same  labor  with  an  expenditure  of  a  pound  and  a  half  of  coal. 

The  average  power  of  an  able-bodied  man  during  his  active 
isequivalentto  life,  supposing  him  to  work  for  twenty  years  at  the  rate  of 
tive  Woweraof  e5gllt  hours  Per  da^'  is  represented  by  an  equivalent  of  about 
a  man  ?  four  tons  of  coal,  since  the  consumption  of  that  amount  will 

evolve  in  a  steam-engine,  fully  as  much  mechanical  force. 
The  great  pyramid  of  Egypt  is  five  hundred  feet  high,  and  weighs  twelve 
thousand  seven  hundred  and  sixty  millions  of  pounds.  Herodotus  states  that 
in  constructing  it  one  hundred  thousand  men  were  constantly  employed  for 
twenty  years.  At  the  present  time,  with  the  consumption  of  480  tons  of 
coal,  all  the  materials  could  be  raised  to  their  present  position  from  the 
ground  in  comparatively  little  time. 

What  is  the  Tlie  £reatest  work  ever  kn(>wn  to  have  been  performed  by 
greatest  amount  a  steam-engine,  was  to  raise  sixty  thousand  tons  of  water  a 
acco^pHslTeT  foot  hi&h  with  the  expenditure  of  one  bushel  of  coal.  This 
by  a  steam-  work  was  accomplished  by  one  of  the  engines  employed  in 
engine  ?  the  ^^  of  Cornwallj  England. 

*  "  Coals  are  by  it  made  to  spin,  weave,  dye,  print,  and  dress  silks,  cottons,  woolens 
and  other  cloths  ;  to  make  paper,  and  print  books  upon  it  when  made  ;  to  convert  corn 
into  flour  ;  to  express  oil  from  the  olive,  and  wine  from  the  grape  ;  to  draw  up  metals 
from  the  bowels  of  the  earth;  to  pound  and  smelt  it;  to  melt  and  mold  it;  to  roll  it 
and  fashion  it  into  every  desirable  form  ;  to  transport,  these  manifold  products  of  its  own 
labor  to  the  doors  of  those  for  whose  convenience  they  are  produced  ;  to  carry  persons  and 
goods  over  the  writers  of  rivers,  lakes,  seas,  and  oceans,  in  opposition  alike  to  the  natural 
difficulties  of  wind  and  water  ;  to  carry  the  wind-bound  ship  out  of  port,  to  place  her  on 
the  open  deep,  ready  to  commence  her  voyage  ;  to  transport  over  the  surface  of  the  sea 
and  the  land,  persons  and  information  from  town  to  town,  and  from  country  to  country, 
with  a  speed  as  much  exceeding  the  ordinary  wind,  as  the  ordinary  wind  exceeds  that  of 
•.pedestrian."—  Lardner.  :: 


252 


WELLS'S   NATURAL   PHILOSOPHY. 


HOW  is  steam         576.  Steam  is  rendered  useful  for  rnechan- 

11  i:id e  available  -.  ,       ,  . 

for  mechanical  ical  purposes  simply  by  its  pressure,  or  elastic 

purposed 


Steam  can  not,  like  wind  and  water,  be  made  to  act  advantageously  by  its 
impulse  in  the  open  air,  because  the  momentum  of  so 
light  a  fluid,  unless  generated  in  vast  quantities,  would 
be  inconsiderable.  The  first  attempts,  however,  to 
employ  steam  as  a  moving  power,  consisted  in  direct- 
ing a  current  of  steam  from  the  mouth  of  a  tube  against 
the  floats  or  vanes  of  a  revolving  wheel. 

A  machine  of  this  kind,  invented  more  than  2,000 
years  ago  by  Hero  of  Alexandria,  is  represented  in 
Fig.  210.  It  consists  of  a  small  hollow  sphere,  fur- 
nished with  arms  at  right  angles  to  its  axis,  and  whose 
ends  are  bent  in  opposite  directions.  The  sphere 
is  suspended  between  two  columns,  bent  and  pointed 
at  their  extremities,  as  represented  in  the  figure :  one 
of  these  is  hollow,  and  conveys  Eteam  from  the  boiler 
below,  into  the  sphere;  and  the  escape  of  the  vapor 
from  the  small  tubes,  by  the  reaction,  produces  a  rotary  motion. 

In  ordor  to  render  the  pressure  of  steam  practically  availa- 
ble in  machinery,  it  is  necessary  that  it  should  be  confined 
within  a  cavity  which  is  air-tight,  and  so  constructed  that  its 
dimensions  or  capacity  can  be  enlarged  or  diminished  without 
impairing  its  tightness.  When  the  steam  enters  such  a  ves- 
sel, its  elastic  force  pressing  against  some  movable  part,  causes 
it  to  recede  before  it,  and  from  this  movable  part  motion  is  communicated  to 
machinery. 

The  practical  arrangement  by  which  such  a     FIG.  211. 
How  are  these  ,  J    . 

conditions  at-      result  is  accomplished  is  by  having  a  hollow 

tained?  cylinder,  A  B,  Fig.  211,  with  a  movable  piston, 

D,  accurately  fitted  to  its  cavity.  "When  steam  under  pressure 
in  a  boiler  is  admitted  into  the  cylinder  below  the  piston,  it 
expands,  and  acting  upon  the  under  surface  of  the  piston, 
causes  it  to  rise,  Ming  the  piston-rod  along  with  it 

Suppose,  as  in  Fig.  212,  the  cylinder  to  be  connected  at  the 
bottom  or  side  with  a  pipe,  R,  opening  into  a  steam  boiler,  and 
on  the  other  side  with  a  pipe,  B,  terminating  in  a  vessel  of 
cold  water.  Suppose  the  valve  in  R  to  be  open,  and  that 
in  B  to  be  shut ;  steam  then  passing  into  the  cylinder  from 
the  boiler  will  force  the  piston  up  to  the  top  of  the  cylinder. 
Let  the  valve  in  R  then  be  shut,  and  the  valve  in  B  be 
opened;  the  steam  contained  in  the  cylinder  will  pass  out 
of  the  pipe  B,  and  coming  in  contact  with  cold  water,  in 
the  vessel  connected  with  it,  will  be  condensed,  and  a  vacuum  (on 


To  render  the 
pressure  of 
steam  availa- 
ble in  machin- 
ery, what  con- 
ditions are 
necessary  ? 


THE  ST  HAM-ENGINE. 


253 


FIG.  212. 


beneath  the  piston.  The  pressure  of 
the  atmosphere  then  acting  upon  the 
other  side  of  the  piston,  will  drive 
it  down.  The  position  of  the  valves 
in  R  and  B  being  reversed,  the  piston 
may  be  raised  anew  by  the  admis- 
sion of  more  steam,  to  be  condensed 
in  its  turn,  and  in  this  manner  the 
alternate  motion  may  be  continued 
indefinitely.  The  alternating,  or  re- 
ciprocating motion  of  the  piston,  is 
converted,  by  means  of  a  lever  and  crank  attached  to  the  top  of  the  pis- 
ton-rod, into  a  rotary  motion,  suitable  for  driving- wheels,  shafts,  and  other 
machinery. 

Such  an  arrangement  as  described  constituted  the  first  practical  steam- 
engine.  It  received  the  name  of  the  atmospheric  engine,  from  the  fact  that 
the  pressure  of  the  atmosphere  was  employed  to  press  down  the  piston  after 
it  had  been  elevated  by  the  steam. 

577.  In  modern  engines,  the  pressure  of  the  atmosphere  is 
not  employed  to  drive  the  piston  down.  The  steam  is  ad- 
mitted into  the  cylinder  above  the  piston,  at  the  same  time 
that  it  is  condensed  or  withdrawn  from  below,  and  thus 
exerts  its  expansive  force  in  the  returning  as  well  as  in  the 
ascending  stroke. 

This  results  in  a  great  increase  of  power.  By  the  condensation  or  with- 
drawal of  the  steam,  a  vacuum,  is  created  below  the  piston,  and  the  steam 
admitted  into  the  cylinder  above  the  piston,  forces  it  through  the  vacuum 
with  an  ease  and  rapidity  far  greater  than  would  be  possible  if  atmospheric 
or  other  resistance  were  to  be  overcome.* 

The  withdrawal  or  condensation  of  the  steam,  in  order  to  produce  a  vacuum 
either  above  or  below  the  piston,  is  accomplished  by  opening  at  the  proper 
time  a  communication  between  the  cylinder  and  a  strong  vessel  situated  at  a 
distance  from  it,  called  the  condenser.  Into  this  vessel  a  jet  of  cold  water  is 
thrown,  which  instantly  condenses  the  steam,  escaping  from  the  bottom  of  the 
cylinder,  into  Water. 


What  is  the 
construction 
and  operation 
of  a  condens- 
ing steam-en- 
gine? 


*  "A  proof  of  the  extraordinary  power  obtained  in  this  way,  through  the  combustion 
of  fuel,  is  presented  in  the  following  calculations:—  One  cubic  inch  of  water  is  converti- 
ble  into  steam,  of  one  atmospheric  pressure  by  15J  grains  of  coal,  and  this  expansion  of 
the  water  into  steam  is  capable  of  raising  a  weight  of  one  ton  the  height  of  a  foot.  The 
one  cubic  inch  of  water  becomes  very  nearly  one  cubic  foot  of  steam,  or  1,728  cubic  inches. 
When  a  vacnum  is  produced  by  the  condensation  of  this  steam,  a  piston  of  one  sqaare 
inch  surface,  that  may  hurc  been  lifted  1,T28  inches,  or  144  feet,  will  fall  with  a  velocity  of  a 
heavy  body  rushing  by  gravity  down  a  perpendicular  height  of  18,500  feet.  This  would 
give  the  falling  body  a  velocity,  at  the  termination  of  its  descent,  equal  to  1,300  feet  per 
second,  greater  than  that  of  the  transmission  of  sound.  From  this  we  can  form  some 
estimate  of  the  strength  of  the  tempest  which  alternately  blows  the  piston  in  its  cylinder, 
when  elastic  steam  of  high-pressure  is  employed.11 — Prof.  H.  D.  Rogers. 


254 


WELLS'S   NATURAL   PHILOSOPHY. 


A  steam-engine  of  this  character  is  called  a  condensing  steam-engine,  be- 
cause the  steam  which  has  been  employed  in  raising  or  depressing  the  piston 
is  condensed,  after  it  has  accomplished  its  object,  leaving  a  vacuum  above  or 
below  the  piston.  It  is  also  called  a  low-pressure  engine,  because,  on  ac- 
count of  the  vacuum  which  is  produced  alternately  above  and  below  the 
piston,  the  steam,  in  acting,  does  not  expend  any  force  in  overcoming  the 
pressure  of  the  atmosphere.  Steam,  therefore,  may  be  used  under  such  condi- 
tions of  low  expansive  force,  or,  as  it  is  technically  called,  of  "low-pressure." 

The  practical  construction  of  the 

piston  and  cylinder,  and  the  ar-  FiCk  213.. 

rangement  of  connecting  pipes  by 
which  the  steam  is  admitted  alter- 
nately above  and  below  the  piston, 
is  fully  shown  in  Fig.  213.  The 
valves,  which  are  of  various  forms, 
are  connected  by  levers  with  the 
machinery,  in  such  a  way  as  to 
open  and  close  with  great  ac- 
curacy at  exactly  the  proper  mo- 
ment. 


Whatisahigh-  ' 

pressure     .en-     engines,  the  appa- 

^ne?  ratus  for  condens- 

ing the  steam  alternately  above 

or  below  the  piston,  is  dispensed 

with,  and  the  steam,  after  it  has 

moved  the  piston  from  one  end  of 

the  cylinder  to  the  other,  is  al- 

lowed to  escape,  by  the  opening  of 

a  valve,  directly  into  the  air.     To 

accomplish  this,  it  is  evident  that 

the  steam  must  have  an  elastic 

force  greater  than  the  pressure  of 

the  atmosphere,  or  it  could  not 

expand  and  drive  out  the  waste 

steam  on  the  other  side  of  the  piston,  in  opposition  to  the  pressure  of  the  air. 

An  engine  of  this  character  is  accordingly  termed  a  "high-pressure"  engine. 

High-pressure  engines  are  generally  worked  with  a  pressure  of  from  fifty 
to  sixty  pounds  per  square  inch  of  the  piston  ;  of  this  pressure,  at  least  fifteen 
pounds  must  be  expended  in  overcoming  the  pressure  of  the  atmosphere,  and 
the  surplus  only  can  be  applied  to  drive  machinery. 

One  of  the  most  familiar  examples  of  a  high  pressure  engine  is  the  loco- 
motive used  on  railroads.  The  steam  which  has  been  employed  in  forcing  the 
piston  in  one  direction  is,  by  the  return  movement  of  the  piston,  forced  out  of 
the  cylinder  into  the  smoke-pipe,  and  escapes  into  the  open  air  with  irregular 
puffs. 


THE   STEAM-ENGINE. 


255 


engines 
lated? 


High-pressure  engines  are  generally  used  in  all  situations 
What  are  the  •      •,         •     . 

advantages  and     where  simplicity  and  lightness  are  required,  as  in  the  case  of 

disadvantages       ^Q  ]ocomotive ;   also  in  situations  where  a  free  supply  of 
of  high-press- 
ure engines  ?        water  for  condensation  can  not  be  readily  obtained.     As  they 

use  steam  at  a  much  higher  pressure  than  the  condensing  en- 
gines, they  are  more  liable  to  accidents  arising  from  explosions.  High-press- 
ure engines  are  less  expensive  than  low-pressure,  since  all  the  apparatus  for 
condensing  the  steam  is  dispensed  with,  the  only  parts  necessary  being  the 
boiler,  cylinder,  piston,  and  valves. 

When  is  tea  579<  **  is  not  necessarv  in  the  steam-engine  that  the  steam 

said  to  be  used  should  flow  continuously  from  the  boiler  into  the  cylinder 
expansively?  during  the  whole  movement  of  the  piston,  but  it  may  be  cut 
off  before  it  has  fully  completed  its  ascent  or  descent  in  the  cylinder.  The 
steam  already  in  the  cylinder  immediately  expands,  and  completes  the  move- 
ment already  begun,  thus  saving  a  considerable  quantity  of  steam  at  each 
movement.  Steam  employed  in  this  way  is  said  to  be  used  expansively. 

To  carry  out  this  plan  to  the  best  advantage,  the 
expansive  force  of  the  steam  must  be  greatly  in- 
creased by  working  it  under  a  high  pressure. 

580.  In  many  engines  the  supply 
How  ia  the  mo-  ,  KF  J 

tion  of  steam-      of  steam  to  the  cylinder  is  regu- 

regu"  lated  by  an  apparatus  called  the 
Governor.  This  consists,  as  is  rep- 
resented in  Fig.  214,  of  two  heavy  balls,  C  and  C', 
connected  by  jointed  rods,  D  D',  with  a  revolving 
axis.  A.  When  the  axis  is  made  to  revolve  rap- 
idly, the  centrifugal  force  tends  to  make  the  balls 
diverge,  or  separate  from  one  another  in  the  same 
mariner  as  the  two  legs  of  a  tongs  will  fly  apart 
when  whirled  round  by  the  top.  This  divergence 
draws  down  the  jointed  rods,  but  a  slower  motion  of  the  axis  causes  the 
balls,  on  the  contrary,  to  approach  each  other,  and  thus  push  them  up. 
These  movements  of  the  jointed  rods  in  turn  raise  or  lower  the  end  of  a  bar, 
E,  which  acts  as  a  lever,  and  moves  a  valve  which  increases  or  diminishes  the 
quantity  of  steam  admitted  from  the  boilers  into  the  cylinder — thus  preserv- 
ing the  motion  of  the  engine  uniform. 

In  stationary  engines,  also,  a  large  and  heavy  fly-wheel  is  often  used,  which 
by  its  momentum  causes  the  machinery  to  move  uninterruptedly,  even  if  the 
pressure  of  steam  be  less  at  one  point  than  at  another* 

•  Fig.  215  illustrates  the  principal  parts  of  a  condensing  steam-engine  and  its  mode  of 
action. 

Upon  the  left  of  the  figure  is  the  cylinder,  which  receives  the  steam  from  the  boiler. 
A  part  of  the  side  of  the  cylinder  is  cut  away  in  order  to  show  the  piston,  which  moves 
alternately  up  and  down  according  as  the  steam  is  admitted  above  or  below  it.  By  the 
rod  A  the  piston  transmits  its  alternating  movements  to  the  walking-beam,  L,  which  is  an 
enormous  lever  accurately  balanced  on  its  center,  and  supported  by  four  columns.  The 
walking-beam,  L,  communicates  its  motion  by  means  of  a  connecting-rod,  I,  to  thecrauk, 


256 


WELLS'S   NATURAL   PHILOSOPHY. 

PiG.  215. 
li 


581.  Steam-boilers,  which,  although  necessary  to  the  generation  of  the 
power,  are  quite  independent  of  the  engine,  are  constructed  of  thick  sheets 
of  iron  or  copper,  strongly  riveted  together. 

K,  by  which  a  rotary  movement  is  communicated  to  the  wheel,  V ;  from  this  the  power 
may  be  applied  by  other  wheels,  or  by  bands  and  pulleys,  to  effect  different  operations. 

At  the  left  of  the  cylinder  is  an  arrangement  of  valves  and  pipes,  by  which  the  steam  is 
allowed  to  act  alternately  above  and  below  the  piston.  After  the  Bteum  has  completed  its 
action  by  forcing  the  piston  to  the  extremity  of  the  cylinder,  it  is  necessary  that  it  should 
be  withdrawn,  and  a  vacuum  formed  in  its  place.  In  order  to  accomplish  this,  the  steam, 
,. After  having  acted,  is  caused  to  pass  into  the  cylinder,  O,  which  contains  cold  water,  and 
is  termed  the  condenser.  Here  it  is  condensed,  and  a  vacuum  formed  in  the  cylinder 
above  or  below  the  piston,  as  the  case  may  be. 

As  the  cold  water  of  the  condenser  becomes  quickly  heated  by  the  condensed  steam 
withdrawn  from  the  cylinder,  it  becomes  necessary  to  constantly  withdraw  the  hot  water 
and  replace  it  by  cold  water,  in  order  that  the  condensation  of  the  steam  may  take  place 
;\s  rapidly  as  possible.  This  is  effected  by  means  of  two  pumps  ;  the  one,  F  M,  which  is 
called  the  "  air-pump,"  which  withdraws  the  hot  water  from  the  condenser,  and  with  it 
any  air  that  may  be  present  either  in  the  cylinder  or  the  condenser ;  the  other,  H  R, 


THE    STEAM-ENGINE.      ^  257 

The  essential  requisites  of  a  steam-boiler  are,  that  it  should 
What  are  the 
essential   req-      possess  sufficient  strength  to  resist  the  greatest  pressure  which 

uisites  of  a.  jg  ever  ijable  to  occur  from  the  expansion  of  the  steam,  and 
that  it  should  offer  a  sufficient  extent  of  surface  to  the  tire 
to  insure  the  requisite  amount  of  vaporization.  In  common  low-pressure 
boilers,  it  requires  about  eight  square  feet  of  surface  of  the  boiler  to  be  ex- 
posed to  the  action  of  the  fire  and  flame  to  boil  off  a  cubic  foot  of  water  in  an 
hour ;  and  a  cubic  foot  of  water  in  its  convertion  into  steam  equals  one- 
horse  power. 

T"he  strongest  form  for  a  boiler,  and  one  of  the  earliest  which  was  used,  is 
that  of  a  sphere ;  but  this  form  is  the  one  which  offers  least  surface  to  the 
fire.  The  figure  of  a  cylinder  is  on  many  accounts  the  best,  and  is  now  ex- 
tensively used,  especially  for  engines  of  high-pressure.  It  has  the  advantage 
of  being  easily  constructed  from  sheets  of  metal,  and  the  form  is  of  equal 
strength  except  at  the  ends.  In  such  a  boiler  the  ends  should  be  made 
thicker  than  the  other  parts. 

called  the  "cold-water  pump,"  draws  from  a  well  or  river  the  cold  water  to  supply  the 
place  of  the  heated  water  withdrawn  from  the  condenser  by  the  air-pump.  There  is  also 
a  third  pump,  G  Q,  which  is  called  the  "  supply"  or  "  feed-pump,"  because  it  pumps  into 
the  boiler  the  hot  water  which  the  air-pump  withdraws  from  the  condenser,  thus  econ- 
omizing the  consumption  of  fuel. 

The  various  parts  of  the  engine  (as  shown  in  Fig.  215)  are  illustrated  in  detail  by  the 
following  descriptive  explanation  : —  4 

A — Piston-rod  connected  wijth  the  walking-beam,  and  transmitting  to  it  the  alternating 
movement  of  the  piston. 

B,  C,  D,  E — Arrangements  of  levers  and  joints,  intended  to  guide  and  preserve  the  pis- 
ton-rod A  in  a  perfectly  rectilinear  track  during  its  up-and-down  movements. 

F — Arm  or  rod  of  the  air-pump,  which  removes  the  hot  water  and  air  from  the  con- 
denser. 

G — Rod  of  the  "  supply"  or  "  feed-pump,"  which  supplies  to  the  boiler  the  hot  water 
withdrawn  from  the  condenser. 

H — Rod  of  the  cold-water  pump,  which  supplies  the  cold  water  necessary  for  con- 
densation. 

I — Connecting-rod,  which  transmits  the  motion  of  the  walking-beam,  L,  to  the  crank,  K. 

M— Cylinder  of  the  air-pump  in  communication  with  the  condenser,  O. 

O — Condenser  filled  with  cold  water,  in  which  the  steam  after  acting  upon  the  piston  is 
condensed. 

P — Piston,  movable  in  the  cylinder ;  it  receives  directly  the  pressure  of  the  steam  upon 
the  upper  and  lower  surface  alternately,  and  transmits  its  movements  by  means  of  the  rod 
A  to  the  rest  of  the  machinery. 

S — Pipe  conducting  the  hot  water  withdrawn  from  the  condenser  to  the  boiler. 

T— Pipe  discharging  the  cold  water  from  the  cold-water  pump  into  the  condenser,  O. 

U— Pipe  conducting  the  steam  from  the  cylinder,  after  it  has  acted  upon  the  piston,  into 
the  condenser. 

V— Fly-wheel. 

Z— Cornecting-rod,  which  transmits  the  movements  of  the  eccentric,  «,  through  the 
lever,  Y,  to  the  valves,  b.  The  eccentric  is  a  wheel  fixed  upon  the,  crank-shaft,  as  seen 
at  e.  It  is  called  an  eccentric  from  the  circumstance  of  the  wheel  not  being  concentric,  or 
having  a  common  center  with  the  crank-shaft  upon  which  it  is  fixed,  It  becomes,  there- 
fore, a  substitute  for  a  short  crank,  and  transmits  a  reciprocating  movement  to  the  rod 
Z,  which  is  connected  with  the  valves  at  6  by  the  lever  Y.  These  valves  being  alternately 
opened  and  closed  by  the  movement  of  the  rod  Z,  admit  the  steam  alternately  above  or 
below  the  piston.  .  / 


258 


WELLS'S  NATURAL   PHILOSOPHY. 


a    locomotive- 
boiler  ? 


FIG.  217. 


What    is    the  ^    V6ry    great    imProvement   was  FlG-  216- 

construction  of  effected  in  the  construction  of  steam- 
a  flue-boiler?  boilers  by  placing  a  cylindrical  fur- 
nace within  a  cylindrical  boiler,  thus  surrounding  the 
heated  surfaces  with  water  upon  all  sides.  By  this 
method,  all  the  heat,  except  what  escapes  up  the 
chimney,  is  communicated  to  the  water.  Such  boilers 
are  known  as  "flue-boilers."  Their  general  form  and 
plan  of  construction  are  represented  in  Fig.  216. 

__   .  The  requirements  of  a  boiler  suit- 

Wnat  are  the 

peculiarities  of     able    for    a    locomotive    are,    that 

the  greatest  possible  quantity  of  water  should  be  evapor- 
ated with  the  greatest  rapidity  in  the  least  possible  space. 
The  quantity  of  fuel  consumed  is  a  secondary  consideration,  as  this  can  be 
carried  in  a  separate  vehicle.  The  principle  by  which  this  has  been  accom- 
plished, and  the  invention  of  which  may  be  said  to  have  made  the  present 
railway  system,  consists  in  carrying  the  hot  product  of  the  fire  through  the 
water  in  numerous  small  parallel  flues  or  tubes,  thus  dividing  the  heated 
matter,  and  as  it  were  filtering  it  through  the  water  to  be  heated.  In  this 
manner  the  surfaces,  by  which  the  water  and  the  heating  gases  communicate, 
are  immensely  increased,  the  whole  having  a  resemblance  to  the  mechan- 
ism of  the  lungs  of  animals,  in 
which  the  air  and  the  blood  are 
divided  and  presented  to  each 
other  at  as  many  points,  and 
with  as  little  intervening  matter 
between  them,  as  is  consistent 
with  their  separation.  Fig.  217 
represents  the  interior  of  the  lire- 
box  of  a  locomotive,  showing 
the  opening  of  the  tubes,  which 
extend  through  the  whole  length 
of  the  boiler,  and  are  surrounded 
with  water.  The  smoke  and 
other  products  of  combustion  pass 
through  these  tubes,  and  finally 
escape  up  the  smoke-pipe.  It 
will  be  further  observed  by  the 
examination  of  the  figure  that 
the  fire-box  is  double- walled,  or  rather  walled  and  roofed  with  a  layer  of 
water,  leaving  onlj*  the  bottom  vacant,  which  receives  the  grate-bars. 

582.  The  safety-valve  is  generally  a  conical  lid  fitted 
into  the  boiler,  and  opening  outward ;  it  is  kept  down  by  a 
weight,  acting  on  the  end  of  a  lever,  equal  to  the  pressure 
which  the  boiler  is  capable  of  sustaining  without  danger  from  the  steam 
generated  within.  If  the  amount  of  steam 'at  any  time  exceeds  the  pressure, 


Describe     the 
safety-valve. 


THE    STEAM-ENGINE. 


259 


FIG.  218. 


How  does  a 
diminution  of 
water  in  boil- 
ers often  oc- 
casion explo- 
sions? 


it  overcomes  the  resistance  of 
the  weight,  lifts  the  valve,  and 
allows  the  steam  to  escape. 
When  sufficient  steam  has 
escaped  to  diminish  the  pres- 
sure, the  valve  falls  back  into 
its  place,  and  the  boiler  is  as 
tight  as  if  it  had  no  such  opening. 

Fig.  218  represents  the  ordinary  construction  of  the  safety-valve. 

583.  The  explosion  of  steam-boilers,  when  the  safety-valv» 
is  in  good  condition  and  working  order,  is  sometimes  inex- 
plicable ;  but  explosions  often  result  from  the  engineer  allow- 
ing the  water  to  become  too  low  in  the  boilers.  When  this 
occurs,  the  parts  of  the  boiler  which  are  not  covered  with 
water,  and  are  exposed  to  the  fire,  become  highly  overheated.  If,  in  this 
condition,  a  fresh  supply  of  water  is  thrown  into  the  boiler,  it  comes  suddenly 
into  contact  with  an  intensely-heated  metal  surface,  and  an  immense  amount 
of  steam,  having  great  elastic  force,  is  at  once  generated.  In  this  case  the 
boiler  may  burst  before  the  inertia  of  the  safety-valve  is  overcome,  and  the 
stronger  the  boiler  the  greater  the  explosion. 

What  is  &  584-  The  degree  of  pressure  which  the  steam  exerts  upon 
steam-guage?  the  interior  of  the  boiler,  and  which  is  consequently  avail- 
able for  working  the  engine,  is  indicated  by  means  of  an  instrument  called 
the  "steam"  or  "barometer-guage."  It 
consists  simply  of  a  bent  tube,  A,  C,  D, 
E,  Fig.  219,  fitted  into  the  boiler  at  one 
end,  and  open  to  the  air  at  the  other. 
The  lower  part  of  the  bend  of  the  tube 
contains  mercury,  which,  when  the  pres- 
sure of  steam  in  the  boiler  is  equal  to 
that  of  the  external  atmosphere,  will 
stand  at  the  same  level,  H  R,  in  both  legs 
of  the  tube.  When  the  pressure  of  the 
steam  is  greater  than  that  of  the  atmos- 
phere, the  mercury  is  depressed  in  the 
leg  C  D,  and  elevated  in  the  leg  D  E.  A 
scale,  G,  is  attached  to  the  long  arm  of 
the  tube,  and  by  observing  the  difference 
of  the  levels  of  the  mercury  in  the  two 
tubes,  the  pressure  of  the  steam  may 

be  calculated.  Thus,  when  the  mercury  is  at  the  same  level  in  both 
legs,  the  pressure  of  the  steam  balances  the  pressure  of  the  atmosphere, 
and  is  therefore  15  pounds  per  square  inch.  If  the  mercury  stands  30 
inches  higher  in  the  long  arm  of  the  tube,  then  the  pressure  of  the  steam 
is  equal  to  that  of  two  atmospheres,  or  is  30  pounds  to  the  square  inch,  and 
so  on. 


FIG.  219. 


260  WELLS'S   NATURAL   PHILOSOPHY. 

H  w    an  th  "^S  ^e  Pressure  °f  steam  increases  with  its  temperature,  the 

pressure  of  pressure  upon  the  interior  of  the  boiler  may  also  be  known  by 
dicated  ^y^a  means  °*  a  thermometer  inserted  into  the  boiler.  Thus  it  has 
thermometer?  been  ascertained  that  steam  at  212°  balances  the  atmosphere, 
or  exerts  a  pressure  of  15  pounds  per  square  inch  ;  at  250°, 
30  pounds;  at  275°,  45  pounds;  at  294°,  60  pounds,  and  so  on. 

585.  The  steam-whistle  attached  to  locomotive  and  other 
Describe     the  .  . 

Kteam-whistie.      engines  is  produced  by  causing  the  steam  to  issue  from  a 

narrow  circular  slit,  or  aperture,  cut  in  the  rim  of  a  metal  cup ; 
directly  over  this  is  suspended  a  bell,  formed  like  the  bell  of  a  clock.  The 
steam  escaping  from  the  narrow  aperture,  strikes  upon  the  edge  or  rim  of  the 
bell,  and  thus  produces  an  exceedingly  sharp  and  piercing  sound.  The  size 
of  the  concentric  part  whence  the  steam  escapes,  and  the  depth  of  the  bell 
part,  and  their  distance  asunder,  regulate  the  tones  of  the  whistle  from  a 
shrill  treble  to  a  deep  bass. 


SECTION    Y .. 

WARMING     AND     VENTILATION. 

upon  what  586.   In    the  warming   and  ventilation   of 

fhenciwarming  buildings,  the  entire  process,  whatever  expe- 
Sfdvbuiwtags  dients  mav  be  adopted,  is  dependent  upon  the 
depend?  expansion  and  contraction  of  air  ;  or  in  other 

words,  upon  the  fact  that  air  which  has  heen  heated  and 
expanded  ascends,  and  air  which  has  been  deprived  of 
heat,  or  contracted,  descends. 

what  is  ven-         587.  Ventilation  is  the  act  or  operation  of 
tiiation?        causing  air  to  pass  through  any  place,  for  the 
purpose  of  expelling  impure  air  and  dissipating  noxious 
vapors. 

The  theoretical  perfection  of  ventilation  is  to  render  it  impossible  for  any 
portion  of  air  to  be  breathed  twice  in  the  same  place. 

In  the  open  air,  ventilation  is  perfect,  because  the  breath,  as 
tiiation  perfect  ?    it  leaves  the  body,  -is  warmer  and  lighter  than  the  surround- 
ing fresh  air,  and  ascending,  is  immediately  replaced  by  an 
ingress  of  fresh  air  ready  to  be  received  by  the  next  respiration. 

.       .  Common  air  consists  of  a  mixture  of  two  gases,  oxygen  and 

once  respired  nitrogen,  in  the  proportion  of  one  fifth  oxygen  to  four  fifths 
unwholesome  ?  nitrogen.  By  all  the  forms  of  respiration  or  breathing,  and 
of  combustion,  the  quantity  of  oxygen  in  atmospheric  air  is  diminished  and 
impaired,  and  to  exactly  the  same  extent  is  air  rendered  unwholesome  and 
unsuitable  to  supply  the  wants  of  the  animal  system. 


WARMING   AND    VENTILATION. 


261 


ftov     mach 
frcwA  air  is  re- 
quired per  hour 
by    a    healthy 
man  ? 


It  is  calculated  that  a  full-grown  person  of  average  sfze  ab- 
sorbs about  a  cubic  foot  of  oxygen  per  il^rDyreSpiration, 
and  consequently  renders  five  cubic  feet  of  air  unlit  for  breath- 
ing, since  every  five  cubic  feet  of  air  contain  one  cubic  foot  of 
oxygen.  It  is  also  calculated  that  two  wax  or  sperm  candles 
absorb  as  much  oxygen  as  an  adult. 

To  render  the  air  of  a  room  perfectly  pure,  five  cubic  feet  of  fresh  air  per 
hour,  for  each  person,  and  two  and  a  half  cubic  feet  for  each  candle,  should 
be  allowed  to  pass  in,  and  an  equal  quantity  to  pass  out. 

in  what  man-         588.  From  every  heated  substance,  an  up- 
"dr  ^Stance"    ward  current  of  air  is  continually  rising. 

generate  a  cur-          The  existence  and  force  of  this  upward  current  may  be 
rent  of  air  ?  ,  .       ,  ,.  .  .  J 

shown  in  the  case  of  an  ordinary  stove,  by  attaching  to  the 

side  of  the  pipe  a  wire  on  which  a  piece  of  thick  paper  cut  in  the  form  of  a 


spiral  is  suspended,  as  is  represented  in  Fig.  220.  The 
upward  current  of  hot  air  striking  against  the  surfaces 
of  the  coil  causes  it  to  revolve  rapidly  around  the  wire. 
_„  Apart  from  the  consideration  of  con- 

and  grates  venience,  it  is  necessary  that  stores  and 
fUwr?1  neaf  the  grates'  intended  for  warming,  should  be 
located  as  near  to  the  floor  of  the  room 
as  possible  ;  since  the  heat  of  a  fire  has  very  little  ef- 
fect upon  the  air  of  an  apartment  below  the  level  of 
the  surface  upon  which  it  is  placed. 

589.  When  a  fire  is  lighted  in  a  stove 
Why     does 

smoke  ascend  or  gntte  to  warm  a  room,  the  smoke 
in  a  chimney  ?  an(^  otner  gaseous  products  of  combus- 

FtG.  221. 


-^      2  99 


tion,  being  lighter  than  the  air  of  the  room, 
ascend,  and  soon  fill  the  chimney  with  a 
column  of  air  lighter,  bulk  for  bulk,  than 
a  column  of  atmospheric  air.  Such  a  col- 
umn, therefore,  will  have  a  buoyancy 
proportional  to  its  relative  lightness,  as 
compared  with  the  external  air  and  the 
air  of  the  apartment. 

The  upward  tendency  of  a  column  of 
heated  air  constitutes  the  draft  of  a  chim- 
ney, and  this  draft  will  be  strong  and  ef- 
fective just  in  the  same  proportion  as  the 
column  of  air  in  the  chimney  is  kept 
warm. 

Fig.  221  represents  a  section  of  a  grate 
and  chimney.  C  D  represents  the  light 
_  and  warm  column  of  air  within  the  chim- 
ney, and  A  B  the  cold  and  heavy  column 


262  WELLS'S'  NATURAL   PHILOSOPHY. 

of  air  outside  the  chimney.  The  column  A  B  being  cold  and  heavy  presses 
down,  the  column  C  D  being  light  and  warm  rushes  up,  and  the  greater  the 
difference  between  the  weight  of  these  two  columns,  the  greater  will  be  the 
draft. 

A  chimney  quickens  the  ascent  of  hot  air  by  keeping  a  long 
cWmney°quick-     column  of  it  together.     A  column  of  two  feet  high  rises,  or  is 

en  the  ascent     pressed  up,  with  twice  as  much  force  as  a  column  of  one 

of  a  column  of       /,     ,          ,         .  , .        ,.         „      ,,         ,        ,,         .      , 

hot  air  ?  *°°t,  and  so  in  proportion  for  all  other  lengths — -just  as  two 

or  more  corks,  strung  together  and  immersed  in  water,  tend 
•upward  with  proportionably  more  force  than  a  single  cork. 

In  a  chimney  where  a  column  of  hot  air  one  foot  in  height  is  one  ounce 
lighter  than  the  same  bulk  of  external  cold  air,  if  the  chimney  be  one  hun- 
dred feet  high,  the  air  or  smoke  in  it  is  propelled  upward  with  a  force  of  one 
hundred  ounces. 

TO  what  is  the  1^  ^e  ^YQ  ^e  sufficiently  hot,  the  draft  of 
ch7mney°fpr<>*  ^e  Chimne7  will  be  proportional  to  its  length. 

portionai  ?  -gor  this  reason,  the  chimneys  of  large  manufacturing  estab- 

lishments are  generally  very  high. 

HOW  should  a  A  chimney  should  be  constructed  in  such 
conducted? e  a  way  that  the  flue  or  passage  will  gradually 
contract  from  the  bottom  to  the  top,  being  widest  at  the 
bottom,  and  the  smallest  at  the  top. 

The  reason  of  this  will  be  evident  from  the  following  con- 
chimney  be  siderations : — At  the  base  of  the  chimney,  the  hot  column  of 
exPanded  air  m*  the  entire  passage ;  but  as  the  hot  air 
ascends  it  gradually  cools  and  contracts,  occupying  less  space. 
If,  therefore,  the  chimney  were  of  the  same  size  all  the  way  up,  the  tendency 
would  be,  that  the  cold  external  air  would  rush  down  to  fill  up  the  space  left 
by  the  contraction  of  the  hot  column  of  air.  This  action  would  still  further 
cool  the  hot  air  of  the  chimney  and  diminish  the  draft. 

Some  persons  suppose  that  a  chimney  should  be  made  larger  at  the  top  than 
at  the  bottom,  because  a  column  of  smoke  ascending  in  the  open  air,  ex- 
pands or  increases  in  bulk  as  it  goes  up.  This,  however,  is  owing,  in  great 
part,  to  the  action  of  currents  in  the  air,  and  to  the  fact,  that  a  column  of 
smoke  freely  exposed  to  the  air,  is  more  rapidly  cooled  than  in  a  chimney, 
and  losing  its  ascensional  power,  tends  to  float  out  laterally,  rather  than 
ascend  perpendicularly. 

The  causes  of  "  smoky  chimneys"  are  various. 
A  chimney  may  smoke  for  want  of  a  sufficient  supply  of 
will  a  chimney      air.     If  the  apartment  is  very  tight,   fresh   air  from  without 
will  not  be  admitted  as  fast  as  it  is  consumed  by  the  fire,  and 
in  consequence  a  current  of  air  rushes  down  the  chimney  to  supply  the  defi- 
ciency, driving  the  smoke  along  with  it. 

A  chimney  will  often  smoke  when  the  heat  of  the  fire  is  not  sufficient  to 


WARMING   AND   VENTILATION.  2G3 

rarefy  all  the  air  in  the  chimney  ;  in  such  cases  the  cold  air  (condensed  in  tho 
upper  part  of  the  flue)  will  sink  from  its  own  weight,  and  sweep  the  ascend- 
ing smoke  back  into  the  room. 

When  the  lire  is  first  lighted,  and  the  chimney  is  filled  with  cold  air,  there 
is  often  no  draft,  and  consequently  the  flame  and  smoke  issue  into  the  room. 
This,  in  most  cases,  is  remedied  by  the  action  of  a  "blower." 

A  blower  is  a  sheet  of  iron  that  stops  up  the  space  above 


aeMjf  a  blower?  ^ie  gra*e  ^ars)  and  prevents  any  air  from  entering  the  chim- 
ney except  that  which  passes  through  the  fuel  and  produces 
combustion.  This  soon  causes  the  column  of  air  in  the  chimney  to  become 
heated,  and  a  draft  of  considerable  force  is  speedily  produced  through  tho 
fire.  The  increase  of  draft  increases  the  intensity  of  the  fire. 

Another  frequent  cause  of  smoky  chimneys  is,  that  when  the  tops  are 
commanded  by  higher  buildings,  or  by  a  hill,  the  wind  in  blowing  over  them, 
falls  like  water  over  a  dam,  and  beats  down  the  smoke.  The  remedy  in  such 
cases  is,  either  to  increase  the  height  of  the  chimney,  or  to  fix  a  bonnet  or 
cowl  upon  the  top.  The  philosophy  of  this  last  contrivance  consists  in  the  fact 
that  in  whatever  direction  the  wind  blows,  the  mouth  of  the  chimney  is 
averted  from  it. 

In  a  room  artificially  heated,  there  are  al- 

What     is     the  _     /  '  .      ,. 

motion  of  the   ways  two  currents  of  air  :  one  of  hot  air  now- 

air    in  a  room      .  „     .  ,  ,  n        i  i        • 

artificially  heat-   ing  out  of  the  room,  and  another  of  cold  air 
flowing  into  the  room. 

If  a  candle  be  held  in  the  doorway  of  such  an  apartment,  near  the  floor,  it 
will  be  found  that  the  flame  will  be  blown  inward  ;  but  if  it  be  raised  nearly 
to  the  top  of  the  doorway,  the  flame  will  be  blown  outward.  The  warm  air, 
in  this  case,  flows  out  at  the  top,  while  the  cold  air  flows  in  at  the  bottom. 

590.  An  open  fire-place  differs  greatly  from  a  close  stovo 
stove  differ  in  respect  to  ventilation,  inasmuch  as  the  former  warms  and 
fire*1  lace  °Pln  ventilates  an  apartment,  while  the  latter  only  warms,  and  can 
respect  to  ven-  hardly  be  said  to  contribute  at  all  to  the  ventilation.  In  a 
close  stove,  no  air  passes  through  the  room  to  the  flue  of 
the  chimney,  except  that  which  passes  through  the  fuel,  and  the  quantity 
of  this  is  necessarily  limited  by  the  rate  of  combustion  maintained  in  the 
stove.  In  an  open  fire-place,  a  large  amount  of  air  is  continually  rushing  up 
the  chimney  through  the  opening  over  the  grate,  irrespective  of  what  passes 
through  the  fire  and  maintains  combustion. 

In  summer  time,  when  no  fire  is  made  in  the  chimney,  the  column  of  air 
in  it  is  generally  at  a  higher  temperature  than  the  external  air,  and  a  current 
will  therefore  in  such  case  be  established  up  the  chimney,  so  that  the  fire- 
place will  still  serve,  even  in  the  absence  of  fire,  the  purposes  of  ventilation. 
In  very  warm  weather,  however,  when  the  external  air  is  at  a  higher  tem- 
perature than  the  air  within  the  building,  the  effects  are  reversed  ;  and  the 
air  in  the  chimney  being  cooled,  and  therefore  heavier  than  the  external  air,  a 
downward  current  is  established,  which  produces  in  the  room  the  odor  of  soot. 


264 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  222. 


r. 


best 
ed? 


ventilat- 


Why  are  open 
fire-places  ill 
adapted  for 
heating  ? 


Fig.  222  represents  the  lines  of  the  currents  descend- 
ing the  chimney  and  circulating  round  an  apartment. 

How  is  a  room  A  room  is  wel1  ventilated  b7  opening 
the  upper  sash  of  a  window ;  because 
the  hot  vitiated  air  (which  always  as- 
cends toward  the  ceiling)  can  thus  escape  more  easily. 
If  the  lower  sash  of  the  window  be  also  partially  opened, 
a  corresponding  current  of  cold  air,  flowing  into  the 
room,  is  created,  and  ventilation  will  be  so  effected  more 
perfectly. 

Open  fire-places  are  ill  adapted  for  the 
economical  heating  of  apartments,  be- 
cause the  air  which  flows  from  the  room 
to  the  fire  becomes  heated,  and  passes 
off  directly  into  the  chimney,  without  having  an  oppor- 
tunity of  parting  with  its  heat  for  any  useful  purpose. 
In  addition  to  this,  a  quantity  of  the  air  of  the  room, 
which  has  been  warmed  by  radiation,  is  uselessly  carried 
away  by  the  draft. 

The  advantages  of  a  stove  over  an 
advantages  and    °Pen  fire-place  are  as  follows: 
disad vantages  i.  Being  detached  from  the  walls  of 

the  room,  the  greater  part  of  the  heat 
produced  by  combustion  is  saved.  The  radiated  heat 
being  thrown  into  the  walls  of  the  stove,  they  become  hot,  and  in  turn  radi- 
ate heat  on  all  sides  of  the  room.  The  conducted  heat  is  also  received  by 
successive  portions  of  the  air  of  the  room,  which  pass  in  contact  with  the 
stove. 

2.  The  air  being  made  to  pass  through  the  fuel,  a  small  supply  is  suffi- 
cient to  keep  up  the  combustion,  so  that  little  need  be  taken  out  of  the 
room;  and 

3.  The  smoke,  in  passing  off  by  a  pipe,  parts  with  the  greater  part  of  its 
heat  before  it  leaves  the  room. 

Houses  warmed  by  stoves,  as  a  general  rule,  are  ill-ventilated.  The  air 
coming  in  contact  with  the  hot  metal  surfaces  is  rendered  impure,  which  im- 
purity is  increased  by  the  burning  of  the  dust  and  other  substances  which 
settle  upon  the  stove.  The  air  is,  in  most  cases  also,  kept  so  dry  as  to  ren- 
der it  oppressive. 

591.  The  method  of  warming  houses  by  the  common  hot- 
air  furnace  is  as  follows : — A  stove,  having  large  radiating  sur- 
faces, is  inclosed  in  a  chamber  (generally  of  masonry).     This 
chamber  is  frequently  built  with  double  walls,  that  it  may  be 
a  better  non-conductor  of  heat.      A  current  of  air  from  with- 
out is  brought  by  a  pipe  or  box,  and  delivered  under  the  stove.     A  part  of 
this  air  is  admitted  to  supply  the  combustion ;  the  rest  passes  upward  in  the 
cavity  between  the  hot  stove  and  the  walls  of  the  brick  chamber,  and,  after 


What    is  the 

method  of 

•warming  by 

hot-air  fur- 
naces ? 


WARMING   AND   VENTILATION.  265 

becoming  thoroughly  heated,  is  conducted  through  passages  in  which  its  light- 
ness causes  it  to  ascend,  and  be  delivered  in  any  apartment  of  the  house. 

In  the  construction  and  arrangement  of  a  furnace  for  heat- 
points  are  of  ing,  the  two  points  of  special  importance  are,  to  secure  a  per- 
ancSntKoi-  fect  combustion  of  the  fuel>  and  the  best  possible  transmission 
structionoffur-  of  all  the  heat  formed,  into  the  air  that  is  to  pass  into  the 
naces  ?  rooms  of  the  house. 

The  first  of  these  requisites  is  obtained  by  having  a  good  draft  and  a  fire- 
box which  is  broad  and  shallow,  so  that  the  coal  shall  form  a  thin  stratum 
»nd  burn  most  perfectly. 

The  second  requisite  is  obtained  by  providing  a  great  quantity  of  surface 
in  the  form  of  pipes,  drums,  or  cylinders,  through  which  the  smoke  and  hot 
gases  must  pass  on  their  way  to  the  chimney,  and  to  which  their  heat  will  be 
imparted,  to  bo  in  turn  delivered  to  the  cold  and  pure  air  of  the  rooms  of 
the  house. 

__       .       ,  592.  The  great  advantages  of  heating  by  steam  are,  that 

advantage  of  the  heat  can  be  communicated  for  a  great  distance  in  any  di- 
steam?  by  ration — upward,  downward,  or  horizontally.  As  the  tem- 
perature of  the  heating  surfaces,  when  low-pressure  steam  is 
used,  never  exceeds  212°  P.,  the  air  in  contact  with  them  is  never  contami- 
nated by  the  burning  of  dust,  or  the  abstraction  of  oxygen. 

Under  favorable  circumstances,  one  cubic  foot  of  boiler  will  heat  about 
two  thousand  cubic  feet  of  suitably  inclosed  space  to  a  temperature  of  70°  to 
80°  P. 

what  is  fuel?  ^^'  ^e  aPPty  the  term  fuel  to  any  sub- 
stance which  serves  as  aliment  or  food  for  fire. 
In  ordinary  language  we  mean  by  fuel  the  peculiar  sub- 
stance of  plants,  or  the  products  resulting  from  their  de- 
composition, designated  under  the  various  names  of  wood, 
coal,  &c. 

What     ro  or          Ia  recentl7  cut  wood,  from  one  fifth  to  one  half  of  its  weight 
tion     of  °  the     is  water ;  after  wood  has  been  dried  in  the  air  for  ten  or 

iStSter°?WOOd     twelve  months>  [i  wiu  even  then  contain  from  15  to  25  per 

cent,  of  water. 

The  amount  of  moisture  in  wood  is  greatest  in  the  spring  and  summer,  when 
the  sap  flows  freely  and  the  influence  of  vegetation  is  the  greatest.  "Wood, 
therefore,  is  generally  cut  in  the  winter,  because  at  that  season  there  is  but 
little  sap  in  the  tissues,  and  the  wood  is  drier  than  at  any  other  period. 

Woods  are  designated  as  hard  and  soft.  This  distinction  is 
desiJnateI°°asS  grounded  upon  the  facility  with  which  they  are  worked,  and' 
hard  and  soft  ?  Up0n  their  power  of  producing  heat.  Hard  woods,  as  the 
oak,  beech,  walnut,  elm,  and  alder,  contain  in  the  same  bulk  more  solid  fiber, 
and  their  vessels  are  narrower  and  more  closely  packed  than  those  of  the 
softer  kinds,  such  as  pine,  larch,  chestnut,  etc. 

12 


266  WELLS'S   NATURAL   PHILOSOPHY. 

what  is   the        594.  The  weight  of  wood  varies  greatly  ; 
we?gPhTofwood?  ft'om  forty-four  hundred  pounds  in  a  cord  of 
dry  hickory,  to  twenty-six  hundred  in  a  cord 
of  dry,  soft  maple. 

what  is  the  595.  For  fuel,  the  most  valuable  of  the  com- 
yST/wood  mon  kinds  of  wood  are  the  varieties  of  hickory; 
for  fuel?  after  that,  in  order,  the  oak,  the  apple-tree, 

the  white-ash,  the  dog-wood,  and  the  beech.  The  woods 
that  give  out  the  least  heat  in  burning  are  the  white-pine, 
the  white-birch,  and  the  poplar. 

Is  it  rofitable  ^^'  ^e  remar^  *s  sometimes  made  that  "  it  is  economy  to 
to  burn  green  burn  green  wood,  because  it  is  more  durable,  and  therefore 
in  the  end  more  cheap."  This  idea  is  erroneous.  The  con- 
sumption of  green  wood  is  less  rapid  than  dry,  but  to  produce  a  given  amount 
of  heat,  a  far  greater  amount  of  fuel  must  be  consumed. 

The  evaporation  of  liquids,  or  their  conversion  into  steam,  consumes  or  ren- 
ders latent  a  great  amount  of  caloric.  When  green  wood  or  wet  coal  is  added 
to  the  fire,  it  abstracts  from  it  by  degrees  a  sufficient  amount  of  heat  to  con- 
vert its  own  sap  or  moisture  into  steam  before  it  is  capable  of  being  burned. 
As  long  as  any  considerable  part  of  this  fluid  remains  unevaporated,  tho 
combustion  goes  on  slowly,  the  fire  is  dull,  and  the  heat  feeble. 

597.  Coal  and  hard  wood  are  not  readily  ignited  by  the 
and  hard  woods  blaze  of  a  match,  because  on  account  of  their  density  they  are' 
difficult  to  ig-  rendered  comparatively  good  conductors,  and  thus  carry  off 
match?  the  heat  of  the  kindling  substance,  so  as  to  extinguish  it, 

before  they  themselves  become  raised  to  the   temperature 
necessary  for  combustion. 

Light  fuel,  on  the  contrary,  being  a  slow  conductor  of  heat,  kindles  easily, 
and,  from  the  admixture  of  atmospheric  air  in  its  pores  and  crevices,  burns 
out  rapidly,  producing  a  comparatively  temporary,  though  often  strong  heat. 


CHAPTER    XIII. 

METEOROLOGY. 

what  is  Me-        ^98.   METEOROLOGY  is  that  department  of 
teoroiogy?       physical  science  which  treats  of  the  atmos- 
phere and  its  phenomena,  particularly  in  its  relation  to 
heat  and  moisture. 

599.  By  climate,  we  mean  the  condition  of  a  place  in 


METEOROLOGY.  267 

what  do  we    relation  to  the  various  phenomena  of  the  at- 
™rmnci£atSe    mosphere,  as  temperature,  moisture,  etc.   Thus, 
we  speak  of  a  warm  or  cold  climate,  a  moist 
or  dry  climate,  etc. 

HOW  is  the  600.  The  mean  or  average  temperature  of 
raSirerf'^day  the  ^a7  ™  f°un(l  by  observing  the  thermometer 
found?  a{.  fixe(i  intervals  of  time  during  the  twenty- 

four  hours,  and  then  dividing  the  sum  of  the  tempera- 
tures by  the  number  of  observations. 

h  .   ..  From  such  a  series  of  observations  it  has  been  found  that 

is  the  tempe-     the  lowest  temperature  of  the  day  occurs  shortly  before  sun- 

da'yfhehfghe^t  rise'  and  the  highest  a  few  hours  after  12  at  noon»  somewhat 
and  lowest  ?  later  in  summer  and  somewhat  earlier  in  winter. 

The  mean  annual  temperature  of  any  par- 
ticular location  is  found  by  taking  the  average  of  all  the 
mean  daily  temperatures  throughout  the  year. 

The  mean  daily  temperature  of  any  place  seems  to  vary  in  a  regular  and 
constant  manner,  while  the  mean  annual  temperature  of  the  same  location  is 
very  nearly  a  constant  quantity.  Thus,  by  long  observations  made  in  Phil- 
adelphia, it  has  been  found  that  the  mean  daily  temperature  of  that  locality  is 
one  degree  less  than  the  temperature  at  9  o'clock,  A.  M.,  at  the  same  place ; 
while  the  mean  annual  temperature  of  Paris  varied  only  4°  in  thirteen  years. 

All  the  results  of  observation  seem  to  show  that  the  same  quantity  of  heat 
is  always  annually  distributed  over  the  earth's  surface,  although  unequally — 
that  is  to  say,  the  average  annual  temperature  of  each  place  upon  the  earth's 
surface  is  very  nearly  the  same.  In  our  latitude,  July  is  on  the  average  the 
hottest  month,  and  January  the  coldest ;  and  in-  reference  to  particular  days, 
we  may  on  an  average  consider  the  26th  of  July  as  the  hottest,  and  the  14th 
of  January  as  the  coldest  day  of  the  year  for  the  temperate  zone  of  the  north- 
ern hemisphere. 

HOW  does  tem-  The  average  annual  temperature  of  the  at- 
wltif'tho  \lti-  mosphere  diminishes  from  the  equator  toward 
tude?  either  pole. 

At  the  equator,  in  Brazil,  the  average  annual  temperature  is  84°  Fahren- 
heit's thermometer ;  at  Calcutta,  lat.  22°  35'  N.,  the  annual  temperature  is 
78°  F. ;  at  Savannah,  lat.  32°  5'  N.  the  annual  temperature  is  65°  F. ;  at 
London,  lat.  51°  31'  N.,  the  annual  temperature  is  50°  F. ;  at  Melville 
Island,  lat.  74°  47'  N.,  the  average  annual  temperature  is  1°  below  zero. 
Wh  is  not  th  ^^"  "^  ^e  whole  surface  of  the  earth  were  covered  by 

temperature  of  water,  or  if  it  were  all  formed  of  solid  plane  land,  possessing 
fiiiplthe8  saine  everywhere  the  same  character,  and  having  an  equal  ca- 
latitude  alike  ?  pacity  at  all  places  for  absorbing  and  again  radiating  heat,  the; 


268  WELLS'S  NATURAL  PHILOSOPHY. 

temperature  of  a  place  would  depend  only  on  its  geographical  latitude,  and 
consequently  all  places  having  the  same  latitude  would  have  a  like  climate. 
Owing,  however,  to  various  disturbing  causes,  such  as  the  elevation  and  form 
of  the  land,  the  proximity  of  the  sea,  the  direction  of  the  winds,  etc.,  places 
of  the  same  latitude,  and  comparatively  near  each  other,  have  very  different 
temperatures. 

In  warm  climates  the  proximity  of  the  sea  tends  to  diminish  the  heat;  in 
cold  climates,  to  mitigate  the  cold.  Islands  and  peninsulas  are  warmer  than 
continents ;  bays  and  inland  seas  also  tend  to  raise  the  mean  temperature. 
Chains  of  mountains  which  ward  off  cold  winds,  augment  the  temperature ; 
but  mountains  which  ward  off  south  and  west  winds,  lower  it.  A  sandy  soil, 
which  is  dry,  is  warmer  than  a  marshy  soil,  which  is  wet  and  subject  to  great 
evaporation. 

602.  Air  absorbs  moisture  at  all  tempera- 

What    is    the  ,  .     .  .,      .  .        .   .,  , 

capacity  of  air    tures,   and  retains   it   in  an  invisible  state. 

for  moisture  ?        m-i  •  n  Ji  ,  i«i 

This  power  of  the  air  is  termed  its  capacity 
for  absorption. 

The  capacity  of  air  for  moisture  increases  with  the  tem- 
perature. 

A  volume  of  air  at  32°  can  absorb  an  amount  of  moisture  equal  to  the  hun- 
dred and  sixtieth  part  of  its  own  weight,  and  for  every  27  additional  degrees 
of  heat,  the  quantity  of  moisture  it  can  absorb  at  32°  is  doubled.  Thus  a  body 
of  air  at  32°  F.  absorbs  the  160th  part  of  its  own  weight ;  at  59°  F.,  the  80th ; 
at  86°  F.,  the  40th ;  at  113°  F.,  the  20th  part  of  its  own  weight  in  moisture. 
It  follows  from  this  that  while  the  temperature  of  the  air  advances  in  an  arith- 
metical series,  its  capacity  for  moisture  is  accelerated  in  a  geometrical  series. 

Air  is  said  to  be  saturated  with  moisture 


When     Is    air 
said  to 
rated? 


said  tobesatu-    wnen  fa  contains  as  much  of  the  vapor  ef  water 


as  it  is  capable  of  holding  with  a  given  tem- 
perature. 

"We  say  that  air  is  dry  when  water  evaporates  quickly,  or  any  wetted  sur- 
face dries  rapidly ;  and  that  it  is  damp  when  moistened  surfaces  dry  slowly, 
or  not  at  all,  and  the  slightest  diminution  of  temperature  occasions  a  deposit 
of  moisture  in  the  form  of  mist  and  rain.  These  expressions  do  not,  however, 
convey  altogether  a  correct  idea  of  the  condition  of  the  atmosphere,  since  air 
which  we  term  "  dry,"  may  contain  much  more  moisture  than  that  which  we 
distinguish  as  "  damp."  For  indicating  the  true  condition  of  the  atmosphere 
in  reference  to  moisture,  we  therefore  use  the  terms  "  absolute"  and  "  relative" 
humidity. 

When  we  speak  of  the  absolute  humidity  of  the  air,  we 

by^tfsolut^and    have  reference  to  the  quantity  of  moisture  contained  in  a  given 

relative  humid-     volume.     By  relative  humidity,  we  refer  to  its  proximity  to 

saturation.     Relative  humidity  is  a  state  dependent  upon  the 

mutual  influence  of  absolute  humidity  and  temperature  ;  for  a  given  volume 


METEOROLOGY. 


269 


what  are  Hy- 


of  air  may  be  made  to  pass  from  a  state  of  dampness  to  one  of  extreme  dry- 
ness,  by  merely  elevating  its  temperature,  and  this,  too,  without  altering  the 
amount  of  moisture  it  contains  in  the  least  degree. 

Instruments  designed    for    measuring  the 
quantity  of  moisture  contained  in  the  atmos- 
phere, are  called  HYGROMETERS.* 

Many  organic  bodies  have  the  property  of  absorbing  vapor, 
principle     are     and  thus  increasing  their  dimensions.     Among  such  may  be 
mentioned  hah*,  wood,  whalebone,  ivory,  etc.     Any  of  these 
connected  with   a  mechanical  arrangement  by  which  the 
change  in  volume  might  be  registered,  would  furnish  a  hygrometer. 

A  large  sponge,  if  dipped  in  a  solution  of  salt,  potash,  soda,  or  any  other 
substance  which  has  a  strong  attraction  for  water,  and  then  squeezed  almost 
dry,  will,  upon  being  balanced  in  a  pair  of  scales  suspended  from  a  steady 
support,  be  found  to  preponderate  or  ascend  according  to  the  relative  damp- 
ness or  dryness  of  the  weather. 

The  beard  of  the  wild  oat  may  also  serve  as  a  hygrometer,  as  it  twists 
around,  during  atmospheric  changes  from  dampness  to  dryness. 

If  we  fix  against  a  wall  a  long  piece  of  catgut,  and  hang  a  weight  to  the 
end  of  it,  it  will  be  observed,  as  the  air  becomes  moist  or  dry,  to  alter  in 
length ;  and  by  marking  a  scale,  the  two  extremities  of  which  are  determined 
by  observation  when  the  air  is  very  dry,  and  when  it  is  saturated  with  moist- 
ure, it  will  be  found  easy  to  measure  the  variations. 

An  instrument  called  the  "  Hair  Hygrom-        FIG.  223. 
"Hair      Hy-      eter, "  is  constructed  upon  this  principle.     It 
grometer."  consists  of  a  human  hair,  fastened  at  one 

extremity  to  a  screw  (see  Fig.  223),  and  at  the  other  pass- 
ing over  a  pulley,  being  strained  tight  by  a  silk  thread  and 
weight,  also  attached  to  the  pulley.  To  the  axis  of  the 
pulley  an  index  is  attached,  which  passes  over  a  graduated 
scale,  so  that  as  the  pulley  turns,  through  the  shortening  or 
lengthening  of  the  hair,  the  index  moves.  When  the  in- 
strument is  in  a  damp  atmosphere,  the  hair  absorbs  a  con- 
siderable amount  of  vapor,  and  is  thus  made  longer,  while 
in  dry  air  it  becomes  shorter;  so  that  the  index  is  of 
course  turned  alternately  from  one  side  to  the  other. 

The  instrument  is  graduated  by  first  placing  it  in  air  ar- 
tificially made  as  dry  as  possible,  and  the  point  on  the 
scale  at  which  the  index  stops  under  these  circumstances, 
is  the  point  of  greatest  dryness,  and  is  marked  0.  The 
hygrometer  is  then  placed  in  a  confined  space  of  air,  which 
is  completely  saturated  with  vapor,  and  under  these  cir- 
cumstances the  index  moves  to  the  other  end  of  the  scale : 
this  point,  which  is  that  of  greatest  moisture,  is  marked 

*  Hygrometer,  from  the  Greek  words  vypoy  (moist)  and  nerpov  (measure). 


270  WELLS'S  NATURAL  PHILOSOPHY. 

100.      The  intervening  space  is  then  divided  into  100  equal  parts,  which 
indicate  different  degrees  of  moisture. 

Such  hygrometers  are  not,  however,  considered  as  altogether  reliable. 


SECTION    I. 

PHENOMENA  AND  PRODUCTION  OP  DEW. 


what  is  Dew?  .  Dew  is  the  moisture  of  the  air  con- 

densed by  coming  in  contact  with  bodies  colder 
than  itself. 

what  is  the  604.  The  temperature  at  which  the  conden- 
Dew-Point?  gation  of  moisture  in  the  atmosphere  com- 
mences, or  the  degree  indicated  by  the  thermometer  at 
which  dew  begins  to  be  deposited,  is  called  the  "  Dew- 
Point." 

Is  the  dew-  ^^s  point  is  by  no  means  constant  or  invariable,  since  dew 
point  a  con-  is  only  deposited  when  the  ah*  is  saturated  with  vapor,  and 
stant  one?  t^e  amount  of  moisture  required  to  saturate  air  of  high  tem- 

perature is  much  greater  than  air  of  low  temperature. 

If  the  saturation  be  complete,  the  least  diminution  of  temperature  is  at- 
tended with  the  formation  of  dew  ;  but  if  the  air  is  dry,  a  body  must  be 
several  degrees  colder  before  moisture  is  deposited  on  its  surface  ;  and  indeed 
the  drier  the  atmosphere,  the  greater  will  be  the  difference  between  the  tem- 
perature and  its  dew-point. 

Dew  may  be  produced  at  any  time  by  bringing  a  vessel  of 
Deduction  *of  cold  water  in*0  a  warm  room.  The  sides  of  the  vessel  cool 
dew  be  occa-  the  surrounding  air  to  such  an  extent  that  it  can  no  longer 
time?  **  any  retain  all  its  vapor,  or,  in  other  words,  the  temperature  of  the 
air  is  reduced  below  the  dew-point  ;  dew  therefore  forms  upon 
the  vessel.  A  pitcher  of  water  under  such  circumstances  is  vulgarly  said  to 
"  sweat." 

In  the  same  manner,  moisture  is  deposited  upon  the  windows  of  a  heated 
apartment  when  the  temperature  of  the  external  air  is  low  enough  to  suffi- 
ciently cool  the  glass. 

summer»  and  tho  earth  is  no 


Wh      is    dew 

formed  in  sum-     longer  receiving  new  supplies  of  heat,  its  surface  begins  to 

mer  after  sun-     throvv  off  the  heat  which  it  haa  accumulated  during  the  day 

by  radiation  ;  the  air,  however,  does  not  radiate  its  heat,  and, 
in  consequence,  the  different  objects  upon  the  earth's  surface  are  soon  cooled 
down  from  7  to  25  degrees  below  the  temperature  of  the  air.  The  warm 
vapor  of  the  air,  coming  in  contact  with  these  cool  bodies,  is  condensed  and 
precipitated  as  dew. 

tn  a  clear  summer's  night,  when  dew  is  depositing,  a  thermometer  laid 


PHENOMENA  AND  PKODUCTION  OF  DEW.      271 

upon  the  grass,  will  sink  nearly  20  degrees  below  one  suspended  in  the  air 
at  a  little  distance  above. 

.  All  bodies  have  not  an  equal  capacity  for  radiating  heat, 

stance^  is*  dew    but  some  cool  much  more  rapidly  and  perfectly  than  others. 

deposited  most    Hence  it  follows,  that  with  the  same  exposure,  some  bodies 

.  will  be  densely  covered  with  dew,  while  others  will  remain 

perfectly  dry. 

Grass,  the  leaves  of  trees,  wood,  etc.,  radiate  heat  very  freely :  but  polished 
metals,  smooth  stones,  and  woolen  cloth,  part  with  their  heat  slowly:  the 
former  of  these  substances  will  therefore  be  completely  drenched  with  dew, 
while  the  latter,  in  the  same  situations,  will  be  almost  dry. 

The  surfaces  of  rocks  and  barren  lands  are  so  compact  and  hard,  that  they 
can  neither  absorb  nor  radiate  much  heat ;  and  (as  their  temperature  varies 
but  slightly)  very  little  dew  deposits  upon  them.  Cultivated  soils,  on  the 
contrary  (being  loose  and  porous)  very  freely  radiate  by  night  the  heat  which 
they  absorb  by  day;  in  consequence  of  which  they  are  much  cooled  down, 
and  plentifully  condense  the  vapor  of  the  air  into  dew.  Such  a  condition 
of  things  is  a  remarkable  evidence  of  design  on  the  part  of  the  Creator,  since 
every  plant  and  inch  of  land  which  needs  the  moisture  of  dew  is  adapted  to 
collect  it;  but  not  a  single  drop  is  wasted  where  its  refreshing  moisture  is  not 
required. 

What  circum  605'  Dew  is  Deposited  most  freely  upon  a  calm,  clear  night, 
stances  influ-  since  under  such  circumstances  heat  radiates  from  the  earth 
duction  of  dew-?  mos^  freety>  an(l  ig  l°st  m  space.  On  a  cloudy  night,  on  the 
contrary,  the  deposition  of  dew  is  almost  entirely  interrupted, 
since  the  lower  surfaces  of  the  clouds  turn  back  the  rays  of  heat  as  they 
radiate,  or  pass  off  from  the  earth,  and  prevent  their  dispersion  into  space ; 
the  surface  of  the  earth  is  not,  therefore,  cooled  down  sufficiently  to  chill  the 
vapor  of  the  air  into  dew. 

When  the  wind  blows  briskly,  also,  little  or  no  dew  is  formed,  since  warm 
air  is  constantly  brought  into  contact  with  solid  bodies,  and  prevents  their  re- 
duction in  temperature. 

can  dew  be        ^ew  is  always  formed  upon  the  surface  of 
properly  said  to    ^  material  upon  which  it  is  foiiDcl,  and  does 
not  fall  from  the  atmosphere. 

Other  things  being  equal,  dew  is  most  abundant  in  situations  most  exposed, 
because  the  radiation  of  heat  is  not  arrested  by  houses,  trees,  etc.  Little  dew 
is  ever  observed  in  the  streets  of  cities,  because  the  objects  are  necessarily 
exposed  to  each  other's  radiation,  and  ah  interchange  of  heat  takes  place, 
which  maintains  them  at  a  temperature  uniform  with  the  air. 
Does  dew  form  Dew  rarely  falls  uPon  tne  surface  of  water,  or  upon  ships 
upon  the  sur-  in  mid-ocean.  The  reason  of  this  is,  that  whenever  the 
face  of  water  ?  aqueous  particles  at  the  surface  are  cooled,  they  become  heavier 
than  those  below  them,  and  sink,  while  warmer  and  lighter  particles  rise  to 
the  top.  These,  in  their  turn,  become  heavier,  and  descend;  and  this  pro> 


272  WELLS'S   NATURAL  PHILOSOPHY. 

cess,  continuing  throughout  the  night,  maintains  the  surface  of  the  water  and 
the  air  at  nearly  the  same  temperature. 

Although  dew  does  not  appear  upon  ships  in  mid-ocean,  it  is  freely  depos- 
ited on  the  same  vessels  arriving  in  the  vicinity  of  land.  Thus,  navigators 
who  proceed  from  the  Straits  of  Sunda  to  the  Coromandel  coast,  know  that 
they  are  near  the  end  of  the  voyage  when  they  perceive  the  r.opes,  sails,  and 
other  objects  placed  on  the  deck  become  moistened  with  dew  during  the 
night. 

The  exposed  parts  of  the  human  body  are  never  covered  with  dew,  because 
the  vital  temperature,  varying  from  9G°  to  98°  F.,  effectually  prevents  a  loss 
of  heat  sufficient  for  its  deposition. 

Dew  is  produced  most  copiously  in  tropical  countries,  because  there  is  in 
such  latitudes  the  greatest  difference  between  the  temperature  of  the  day  and 
that  of  the  night.  The  development  of  vegetation  is  also  greatest  in  tropical 
countries,  and  a  great  part  of  the  nocturnal  cooling  is  due  to  the  leaves  which 
present  to  the  sky  an  immense  number  of  thin  bodies,  having  large  surface, 
well  adapted  to  radiate  heat. 

Dew  rarely  falls  upon  the  small  islands  of  the  Pacific ;  the  reason  is,  that 
the  air  over  the  vast  ocean  in  which  these  islands  are  situated,  preserves  a 
nearly  uniform  temperature  day  and  night.  The  islands  are  comparatively 
of  small  extent,  and  the  stratum  of  air  cooled  by  the  contact  of  the  soil  is 
•warmed  by  mixing  with  the  air  that  is  constantly  reaching  it  from  the  sea. 
This  prevents  a  depression  of  temperature  in  the  air  sufficient  to  cause  a  depo- 
sition of  dew. 

what  is  frost?         606.  Frost  is  frozen  dew. 

When  the  temperature  of  the  body  upon  which  the  dew  is 
deposited  sinks  below  32°  F.,  the  moisture  freezes  and  assumes  a  solid  form, 
constituting  what  is  called  "frost." 

Shrubs  and  low  plants  are  more  liable  to  be  injured  by  frost  than  trees  of 
a  greater  elevation,  since  the  air  contiguous  to  the  surface  of  the  ground  is  the 
most  reduced  in  temperature. 

why  does  a  An'  exceedingly  thin  covering  of  muslin, 
proteet°oi5eSf  Batting,  etc.,  will  prevent  the  deposition  of 
frost? dew  °r  dew  or  fr°s*  uPon  an  object,  since  it  prevents 
the  radiation  of  heat,  and  a  consequent  cool- 
ing sufficient  to  occasion  the  production  of  either  dew  or 
frost. 

Fig.  224,  in  which  the  arrows  indicate  the  movements  of  heat,  and  the 
numerals  the  temperatures  of  the  earth  and  air  under  different  circumstances, 
will  render  the  explanations  of  the  phenomena  of  dew  and  frost  more  in- 
telligible. 

The  figures  in  the  middle  of  the  diagram  represent  the  temperature  of  the  air 
at  a  distance  from  the  surface  of  the  earth ;  the  figures  in  the  margin,  the 
of  the  air  adjoining  the  surface  of  the  earth ;  the  figures  below 


CLOUDS,   BAIN,   SNOW,   AND  HAIL. 


273 


the  margin,  the  temperature  of  the  earth  itself.    The  directions  of  the  arrows 
represent  the  radiation  and  reflection  of  the  heat. 

FIG.  224. 


Surface  of 
the  earth,  59°. 

41°. 
Dew. 

32°. 
Frost. 

53°. 
No  dew  or  froat. 

41°. 
No  dew  or  frost. 

In  the  day- 
time. 

In  clear  and  serene 
nights. 

Cloudy  or  windy 
nights. 

Clear  night  ; 
soil  protected. 

What     are 
clouds  ? 


SECTIONIIV 

CLOUDS,   BAIN,    SNOW,   AND  HAIL. 

607.  Clouds  consist  of  vapor  evaporated  from 
the   earth,    and   partially   condensed   in    the 
higher  regions  of  the  atmosphere. 

HOW  is  mist  or  When  air,  saturated  with  vapor,  in  imme- 
fog occasioned?  faaiQ  confcact  with  the  surface  of  the  earth  is 
cooled  down  rapidly,  its  vapor  is  condensed  ;  if  the  con- 
densation, however,  is  not  sufficient  to  allow  of  its  precipi- 
tation in  drops,  it  floats  above  the  surface  of  the  earth  as 
mist  or  fog. 

HOW  do  clouds,        Clouds,  fog,  and  mist  differ  only  in  one  re- 
d°ifferTd  miBt    8Pect-     Clouds  float  at  an  elevation  in  the  air, 
while  fogs  and  mists  come  in  contact  with  the 
surface  of  the  earth. 

Mist  and  fog  are  also  formed  when  the  water  of  lakes  and  rivers,  or  the 
damp  ground,  is  warmer  than  the  surrounding  air  which  is  saturated  with 
moisture.  The  vapors  which  rise  in  consequence  of  the  higher  temperature 
of  the  water,  are  immediately  recondensed,  as  soon  as  they  diffuse  themselves 
through  the  colder  air. 

Mist  and  fog  are  observed  most  frequently  over  rivers  and  marshes,  be- 
in  such  situations  the  air  is  nearly  saturated  with  vapor,  and  therefore 
12* 


274  WELLS'S   NATURAL  PHILOSOPHY. 

the  least  depression  of  temperature  will  compel  it  to  relinquish  some  of  its 
moisture. 

.^  The  moisture  contained  in  the  air  we  expel  from  the  lungs 

moisture  of  in  the  process  of  respiration,  is  visible  in  winter,  but  not  in 
iblebtoa^nter  summer-  Tne  reason  of  this  is,  that  in  cold  weather  the  vapor 
and  not  in  is  condensed  by  the  external  air,  but  in  summer  the  tempera- 
ture of  the  air  is  not  sufficiently  reduced  to  effect  condensation. 
In  -what  man-  During  the  daily  process  of  evaporation  from  the  surface  of 
ner  are  clouds  the  earth,  warm,  humid  currents  are  continually  ascending; 
the  higher  they  ascend,  the  colder  is  the  atmosphere  into 
which  they  enter ;  and  as  they  continue  to  rise,  a  point  will  at  length  be 
attained  where,  in  union  with  the  colder  air,  their  original  humidity  can  no 
longer  be  retained :  a  cloud  will  then  appear,  which  increases  in  bulk  with 
the  upward  progress  of  the  current  into  colder  regions. 

To  a  person  in  the  valley,  the  top  of  a  mountain  may  seem  enveloped  in 
clouds ;  while,  if  he  were  at  the  summit,  he  would  be  surrounded  by  a  mist, 
or  fog. 

The  reason  why  clouds,  which  are  condensed  vapor,  float 

Why  do  clouds  * 

float  iu  the  at-  in  the  atmosphere  is,  that  they  consist  of  very  minute  glob- 
mosphere  ?  u]ea  (cane(j  vesicles),  which,  although  heavier  than  the  sur- 
rounding air,  have  a  great  extent  of  surface  in  comparison  with  their  weight. 
On  account  of  the  resistance  of  the  air,  they  sink  very  slowly,  as  a  soap- 
bubble,  which  greatly  resembles  these  vesicles,  sinks  but  slowly  in  a  calm 
atmosphere.  As  these  vesicles  do,  however,  gradually  sink,  the  question 
arises,  why  do  not  the  clouds  fall  to  the  ground  ?  The  explanation  of  this  is, 
that  the  vesicles  which  sink  in  calm  weather  can  not  reach  the  ground,  be- 
cause in  their  descent  they  soon  meet  with  warmer  strata  of  air  which  are  not 
saturated  with  moisture,  where  they  again  dissolve  into  vapor  and  are  lost  to 
view:  at  the  same  time  that  the  vesicles  of  vapor  dissolve  at  the  lower  limits 
of  the  clouds,  new  ones  are  formed  above,  and  thus  the  cloud  appears  to  float 
immovably  in  the  air. 

When  the  atmosphere  is  agitated,  the  vesicles  of  vapor  constituting  clouds 
are  driven  in  the  direction  of  the  currents  of  air.  A  wind  moving  in  a  hori- 
zontal direction  will  carry  the  clouds  in  the  same  direction ;  and  an  ascend- 
ing current  of  air  will  lift  them  up,  as  soon  as  its  velocity  becomes  greater 
than  the  velocity  with  which  the  vesicles  would  fall  to  the  ground  in  a  calm 
condition  of  the  air.  In  like  manner,  soap-bubbles  are  elevated  by  the  wind 
and  carried  to  considerable  distances. 

Clouds  frequently  appear  and  disappear  with  a  change  in 
affect  the  the  direction  and  character  of  the  wind.  Thus,  if  a  cold  wind 
clouds?  blows  suddenly  over  any  region,  it  condenses  the  invisible  va- 

por of  the  air  into  cloud  or  rain ;  but  if  a  warm  wind  blows  over  any  region, 
it  disperses  the  clouds  by  absorbing  their  moisture. 

The  average  height  at  which  clouds  float  above  the  surface 

°^  *ae  earth  in  a  calm  day,  is  between  one  and  two  miles. 
pt  clouds  ?  Light,  fleecy  clouds,  however,  sometimes  attain  an  elevation 

of  five  or  six  miles. 


CLOUDS,    RAIN,    SNOW,    AND    HAIL.  275 

What  occasions  When  clouds  are  not  continuous  over  the  whole  surface  of 
and  brokenUap-  the  sky,  various  circumstances  contribute  to  give  them  a 
pearance  of  rough  and  uneven  appearance.  The  rays  of  the  sun  falling 
upon  different  surfaces  at  different  angles,  melt  away  one  set 
of  elevations  and  create  another  set  of  depressions ;  the  heat  also,  which  is 
liberated  below  in  the  process  of  condensation,  the  currents  of  warm  air 
escaping  from  the  earth,  and  of  cold  air  descending  from  above,  all  tend  to 
keep  the  clouds  in  a  state  of  agitation,  upheaval,  and  depression.  Under 
these  influences,  the  masses  of  condensed  vapor  composing  the  clouds  are 
caused  to  assume  all  manner  of  grotesque  and  fanciful  shapes. 

The  shape  and  position  of  clouds  is  also  undoubtedly  influenced  in  a  con- 
siderable degree  by  their  electrical  condition. 

Clouds  are  frequently  seen  to  collect  around 

Why  do  clouds  .  ,  ,  •• 

frequently  coi-  mountain  peaks,  when  the  atmosphere  else- 
mountain  where  is  clear  and  free  from  clouds.  This  is 
caused  by  the  wind  impelling  up  the  sides  of 
the  mountains  the  warm,  humid  air  of  the  valleys,  the 
moisture  of  which,  in  its  ascent,  gradually  becomes  con- 
densed by  cold,  and  appears  as  a  cloud. 

608.  Clouds  are   generally  divided  into  four 

How        many  * 

Mndsof  clouds    great  classes,  viz. :  the  CIRRUS,  the  CUMULUS. 

are  recognized  ?        -. 

the  STRATUS,  and  the  NIMBUS. 
SSiSmd1?        The  Cirrus*  cloud  consists  of  very  delicate 

thin  streaks,  or  feathery  filaments,  and  is 
usually  seen  floating  at  great  elevations  in  the  sky  during 
the  continuance  of  fine  weather. 

It  is  highly  probable  that  the  cirrus  cloud,  at  great  elevations,  does  not  con- 
sist of  vesicles  of  mist,  but  of  flakes  of  snow. 

Fig.  225,  a,  represents  the  appearance  of  this  variety  of  cloud. 

What  is  the  The  Cumulus  f  cloud  consists  of  large  round- 
ed masses  of  vapor,  apparently  resting  upon 
a  horizontal  basis.?  When  lighted  up  by  the  SUD,  cu- 
mulus clouds  present  the  appearance  of  mountains  01 
snow. 

The  cumulus  is  especially  the  cloud  of  day,  and  its  figure  is  most  perfect 
during  the  fine,  warm  days  of  summer. 

Fig.  225,  b,  illustrates  the  appearance  of  the  cumulus  cloud. 

These  clouds  appear  in  greatest  number  at  noon,  on  a  fine  day,  but  disap- 
pear as  evening  approaches.  The  explanation  of  this  is,  that  at  noon  the  cur- 

*  From  the  Latin  word  cirrus — a  lock  of  hair,  or  curl, 
t  From  the  Latin  word  cumulus — a  mass,  or  pile. 


276 


WELLS'S   NATURAL   PHILOSOPHY. 


rents  of  warm  air  ascending  from  the  earth  are  more  buoyant,  larger,  and  rise 
higher,  and  when  condensed,  form  large  masses  of  clouds,  each  of  which  may 
be  considered  as  the  capital  of  a  column  of  air,  whose  base  rests  upon  the  earth. 
As  the  heat  of  the  sun  diminishes  in  the  afternoon,  the  strength  of  the  cur- 
rents abate,  the  clouds,  which  are  buoyed  up  by  their  force,  sink  down  into 
warmer  regions  of  the  atmosphere,  and  are  either  partially  or  wholly  dis- 
golved. 

FIG.  225. 


Cirrus,  a;  Cumulus,  b;  Stratus  and  Nimbus,  c. 

The  rounded  figure  of  the  cumulus  has  been  attributed  to  its  method  of 
formation ;  for  when  one  fluid  flows  through  another  at  rest,  the  outline  of 
the  figure  assumed  by  the  first  will  be  composed  of  curved  lines.  This  fact 
may  be  shown,  and  the  appearance  of  the  cumulus  imitated,  by  allowing  a 
drop  of  milk  or  ink  to  fall  into  a  glass  of  water.  The  same  thing  is  also 
seen  hi  the  shape  of  a  cloud  of  steam  aa  it  issues  from  the  boiler  of  a  loco- 
motive. 


CLOUDS,    BAIN,    SNOW,    AND   HAIL.  277 

what  i«  the  The  Stratus/*  or  stratified  cloud,  consists 
stratus  cloud?  Gf  horizontal  streaks,  or  layers  of  vapor,  which 
float  like  a  veil  at  no  very  great  elevation  from  the  surface 
of  the  earth.  They  frequently  appear  with  extraordinary 
brilliancy  of  color  at  sunset. 

The  appearance  of  the  stratus  is  represented  at  c,  Fig.  225. 

what  is  the        ^v  Nimbus,  or  the  cloud  of  rain,  has  no 

Nimbus?       characteristic  form.     It  generally  covers  the 

whole  horizon,  imparting  to  it  a  bluish  black  appearance. 

The  various  forma  of  clouds  gradually  pass  into  each  other,  so  that  it  is 
often  difficult  to  decide  whether  the  appearance  of  a  cloud  approaches  more 
to  one  type  than  another.  The  intermediate  forms  are  sometimes  designated 
as  cirro-stratus,  cirro-cumulus,  and  cumulo-stratus. 

609.  Bain  is  the  vapor  of  the  clouds  or  air 

What  is  Rain?  ,  ,          ,  .     .   r.     ,    .     ,,  ,,    .      , 

condensed  and  precipitated  to  the  earth  in  drops. 
HOW  is  rain  Bain  is  generally  occasioned  by  the  union  of 
occasioned?  ^wo  or  more  volumes  of  humid  air,  differing 
considerably  in  temperature.  Under  such  circumstances, 
the  several  portions  in  union  are  incapable  of  absorbing  the 
same  amount  of  moisture  that  each  could  retain  if  they 
had  not  united.  The  excess,  if  very  great,  falls  as  rain  ; 
if  of  slight  amount,  it  appears  as  cloud, 
upon  what  law  610.  The  law  upon  which  the  condensation 
of  vapor  and  the  formation  of  rain  depends  is, 
f-jjat  the  capacity  of  the  air  for  moisture  de- 
creases in  a  greater  ratio  than  the  temperature. 

611.  Rain  falls  in  drops,  because  the  vesicles  of  vapor,  in 
t^ieir  descent>  attract  each  other  and  merge  together,  thus 
forming  drops  of  water.     The  size  of  the  drop  is  increased  in 
proportion  to  the  rapidity  with  which  the  vapors  are  condensed. 

In  rainy  weather  the  clouds  fall  toward  the  earth,  for  the  reason  that  they 
are  heavy  with  partially-condensed  vapors,  and  the  air,  on  account  of  its 
diminished  density,  is  less  able  to  buoy  them  up. 

612.  The  quantity  of  rain  falling  at  any  one  time  or 
place,  is  measured  by  means  of  an  instrument  called  a 
"  Bain-Guage." 

This  usually  consists  of  a  tin  cylindrical  vessel,  M,  Fig. 
226'  the  uPPer  Part  of  which  is  closed  by  a  cover,  B,  in  the 
shape  of  a  funnel,  with  an  aperture  in  its  center.     The  water 
*  Stratus,  from  the  Latin  stratum — that  which  lies  low  in  the  form  of  a  bed  or  layer. 


278 


WELLS'S  NATURAL  PHILOSOPHY. 


226.  falling  upon  the  top  of 

the  cylinder  flows  into 
the  ulterior  through 
the  opening,  and  i 
thus  protected  from 
evaporation.  From  the 
base  of  the  appara- 
tus a  graduated  glass 
tube,  A,  ascends,  in 
.which  the  water 
rises  to  the  same 
height  as  in  the  in- 
terior of  the  cylinder. 

Supposing  the  apparatus  to  be  placed  in  an  exposed  situation,  and  at  the  end 
of  a  month,  for  example,  the  height  of  the  water  in  the  tube  is  five  inches: 
this  would  indicate  that  the  water  in  the  cylinder  had  attained  to  an  equal 
elevation,  and  consequently  that  the  rain  which  had  fallen  during  this  inter- 
val, would,  if  not  diminished  by  evaporation  or  infiltration,  cover  the  earth  to 
the  depth  of  five  inches. 

613.  Rain  falls  most  abundantly  in  countries  near  the  equa- 
In  whak  situa- 

tions is  rain  tor,  and  decreases  in  quantity  as  we  approach  the  poles. 
most  abundant?  There  are  more  rainv  ^^  howevrerj  m  foe  temperate  zones 
than  in  the  tropics,  although  the  yearly  quantity  of  rain  falling  in  the  latter 
districts  is  much  greater  than  in  the  former. 

In  the  northern  portions  of  the  United  States,  there  are  on  an  average  about 
134  rainy  days  in  a  year  ;  in  the  Southern  States  the  number  is  somewhat 
less,  being  about  103. 

The  reason  why  it  rains  more  frequently  in  the  temperate  zones  than  in  the 
tropics  is  because,  the  former  are  regions  of  variable  winds,  and  the  tempe- 
rature of  the  atmosphere  changes  often  ;  while  in  the  tropics  the  wind  changes 
but  rarely,  and  the  temperature  is  very  constant  throughout  a  great  part  of 
the  year.  In  the  tropics  the  year  is  divided  into  only  two  seasons,  tho  wet, 
or  rainy,  and  the  dry  season. 

The  average  yearly  fall  of  rain  in  the  tropics 
*s  ninety-five  inches  ;  in  the  temperate  zone 
countries?         onty  thirty-five. 

The  greatest  rain-fall,  however,  is  precipitated  in  the  shortest  time.  Ninety- 
five  inches  fall  in  eighty  days  on  the  equator,  while  at  St.  Petersburg  the 
yearly  rain-fall  is  but  seventeen  inches,  spread  over  one  hundred  and  sixty- 
.nine  days.  Again,  a  tropical  wet  day  is  not  continuously  wet.  The  morn- 
ing is  clear  ;  clouds  form  about  ten  o'clock  ;  the  rain  begins  at  twelve,  and 
pours  till  about  half  past  four  ;  by  sunset  the  clouds  are  gone,  and  the  nights 
are  invariably  fine. 

The  depth  of  rain  which  falls  yearly  in  London  is  about  twenty  -five  inches  ; 
but  at  Vera  Cruz,  in  the  Gulf  of  Mexico,  rain  to  the  amount  of  two  hundred 


what  is  the 


CLOUDS,    BAIN,    SNOW,    AND   HAIL.  279 

and  seventy-eight  inches  is  precipitated.  The  explanation  of  this  is  to  be 
found  in  the  peculiar  location  of  the  city,  at  the  foot  of  lofty  mountains,  whose 
summits  are  covered  with  perpetual  snow  ;  against  these  the  hot,  humid  air 
from  the  sea  is  driven  by  the  winds,  condensed,  and  its  excess  of  moisture 
precipitated  as  rain. 

614.  Some  countries  are  entirely  destitute  of  rain  ;  in  a  part  of  Egypt  it 
never  rains,  and  in  Peru  it  rains  once,  perhaps,  in  a  man's  lifetime.  Upon 
the  table-land  of  Mexico,  in  parts  of  Guatemala  and  California,  rain  is  very 
rare.  But  the  most  extensive  rainless  districts  are  those  occupied  by  the 
great  desert  of  Africa,  and  its  continuation  eastward  over  portions  of  Arabia 
and  Persia  to  the  interior  of  Central  Asia,  over  the  great  desert  of  Gobi,  the 
table-land  of  Thibet,  and  part  of  Mongolia.  These  regions  embrace  an  area 
of  five  or  six  millions  of  square  miles  that  never  experience  a  shower. 

The  cause  of  this  scarcity  is  to  be  sought  for  in  the  peculiar  conformation 
of  the  country. 

In  Peru,  for  example,  parallel  to  the  coast,  and  at  a  short  distance  from  the 
sea,  is  the  lofty  range  of  the  Andes,  the  peaks  of  which  are  covered  with 
perpetual  snow  and  ice.  The  prevailing  wind  is  an  east  wind,  sweeping  from 
the  Atlantic  to  the  Pacific  across  the  continent  of  South  America.  As  it  ap- 
proaches the  west  coast,  it  encounters  this  range  of  mountains,  and  becomes 
so  cooled  by  them  that  it  is  forced  to  precipitate  its  moisture,  and  passes  on 
to  the  coast  almost  devoid  of  moisture.  In  Egypt  and  other  desert  countries, 
the  dry  sandy  plains  heat  the  atmosphere  to  such  an  extent  that  it  absorbs 
moisture,  and  precipitates  none. 

On  the  other  hand,  there  are  some  countries  in  which  it  may  be  said  to 
always  rain.  In  some  portions  of  Guiana,  in  South  America,  it  rains  for  a 
great  portion  of  the  year.  The  fierce  heat  of  the  tropical  sun  fills  the  atmos- 
phere with  vapor,  which  returns  to  the  earth  again  in  constant  showers  as 
the  cool  winds  of  the  ocean  flow  in  and  condense  it. 

What  is  the  615.  The  whole  quantity  of  water  annually  precipitated  as 
•whole  esti-  rain  over  the  earth's  surface  is  calculated  to  exceed  seven 


*a        hundred  and  sixty  millions  of  tons.      This  entire  amount  is 
rain?  raised  into  the  atmosphere  solely  by  evaporation.     It  has  been 

also  calculated,  that  the  daily  amount  of  water  raised  by 
evaporation  from  the  sea  alone,  amounts  to  no  less  than  one  hundred  and 
sixty-four  cubic  miles,  or  about  sixty  thousand  cubic  miles  annually. 

During  the  months  of  October  and  November,  the  daily  amount  of  evapo- 
ration from  the  surface  of  the  ocean,  between  the  Cape  of  Good  Hope  and 
Calcutta,  is  known  to  average  three  quarters  of  an  inch  from  the  whole 
surface. 

What  curious  ^e  amoun*  °f  moisture  constantly  present  in  the  atmos- 
influences  are  phere  of  any  country,  exercises  an  important  influence  upon 
thTSture^  the  Physical  system  of  the  inhabitants,  and  upon  their  arts 
the  atmosphere?  and  professions.  The  atmosphere  of  the  northern  United 

States  is  uncommonly  dry,  much  more  so  than  in  England  or 
Germany.  To  this  in  a  great  measure  is  owing  the  difference  in  the  physical 


280  WELLS'S  NATURAL  PHILOSOPHY. 

appearance  of  the  inhabitants  of  these  respective  countries.  Painters  find  that 
their  work  dries  quicker,  also,  in  New  England  than  in  central  Europe. 
Cabinet-makers  in  the  United  States  are  obliged  to  use  thicker  glue,  and 
watchmakers  animal  instead  of  vegetable  oil.  Pianos  are  rarely  imported  from 
Europe  into  the  United  States,  because  the  difference  in  the  climate  of  these 
two  countries  is  so  great,  as  respects  moisture,  that  the  foreign  instruments 
shrink,  and  quickly  become  damaged. 

what  is  snow?        616.  Snow  is  the  condensed  vapor  of  the  air, 
frozen  and  precipitated  to  the  earth. 

How  is  snow  Our  knowledge  ia  respect  to  the  formation  of  snow  in  the 
probably  form-  atmosphere  is  very  limited.  It  is  probable  that  the  clouds  in 
which  the  flakes  of  snow  are  first  formed,  consist,  not  of  vesi- 
cles of  vapor,  but  of  minute  crystals  of  ice,  which  by  the  continuous  condens- 
ation of  vapor  become  larger  and  form  flakes  of  snow,  which  continue  to 
increase  in  size  as  they  descend  through  the  air. 

"When  the  lower  regions  of  the  air  are  sufficiently  warm,  the  flakes  of  snow 
melt  before  they  reach  the  ground ;  so  that  it  may  ram  below,  while  it  snows 
above. 

The  largest  flakes  of  snow  are  formed  when  the  air  abounds  with  vapor, 
and  the  temperature  is  about  32°  F. ;  but  as  the  moisture  diminishes,  and  the 
cold  increases,  the  snow  becomes  finer. 

In  extreme  cold  weather,  when  a  volume  of  cold  air  is  suddenly  admitted 
into  a  room,  the  air  of  which  is  saturated  with  moisture,  it  sometimes  hap- 
pens that  the  vapor  of  the  room  will  be  condensed  and  frozen  at  the  same 
instant,  thus  producing  a  miniature  fall  of  snow. 

.      .  617.  On  examining  a  snow-flake  beneath  a  microscope,  it  is 

physical  com-  found  to  consist  of  regular  and  symmetrical  crystals,  having  a 
S^Sk.1*  great  diversity  of  form. 

These  crystals  also  exist  in  ice,  but  are  so  blended  together 
that  their  symmetry  is  lost  in  the  compact  mass. 

Th*e  crystals  of  snow  may,  under  favorable  circumstances,  be  seen  with 
the  naked  eye,  by  placing  the  flake  upon  a  dark  body  cooled  below  32°  F. 
Fig.  227  represents  the  varied  and  beautiful  forms  of  snow  crystals. 

The  bulk  of  recently-fallen  snow  is  ten  or  twelve  times  greater  than  that 
of  the  water  obtained  by  melting  it. 

618.  Hail  is  the  moisture  of  the  air  frozen 

What  is  Hail?       .  .  ~  . 

into  drops  of  ice. 

Can  the  phe-  Tbe  phenomenon  of  hail  has  never  been  satisfactorily  ex- 
nomenonofbail  plained.  It  is  difficult  to  conceive  how  the  great  cold  is  pro- 
satisfactorily  ?  duced  which  causes  the  water  to  freeze  under  the  circum- 
stances, and  also  how  it  is  possible  that  the  hail-stones,  after 
having  once  become  sufficiently  large  to  fall  by  their  own  weight,  can  yet 
remain  long  enough  in  the  air  to  increase  to  so  considerable  a  size  as  ia 
sometimes  seen.  A  hail-storm  generally  lasts  but  a  few  minutes,  very  sel- 
dom as  long  as  a  quarter  of  an  hour;  but  the  quantity  of  ice  which 


WINDS. 


281 


escapee  from  the  clouds  in  so  short  a  time  is  very  great,  and  masses  have 
been  observed  to  fall  of  a  weight  of  10  or  12  ounces. 


.  22?. 


619.  Hail-stones  are  generally  pear-shaped,  and  if  they  are  divided  through 
the  center,  they  will  be  found  to  be  composed  of  alternate  layers  of  ice  aiia 
snow,  arranged  around  a  nucleus,  like  the  coats  of  an  onion. 

Hail-storms  occur  most  frequently  in  temperate  climates,  and  rarely  within 
the  tropics.  They  occur  most  frequently  in  northern  latitudes,  in  the  vicinity 
of  high  mountains,  whose  peaks  are  always  covered  with  ice  and  snow.  The 
south  of  France,  which  lies  between  the  Alps  and  Pyrenees,  is  annually  rav- 
aged by  hail ;  and  the  damage  which  it  causes  yearly  to  vineyards  and  stand- 
ing crops  has  been  estimated  at  upward  of  nine  millions  of  dollars. 


SECTION    III. 


WINDS. 


What  is  Wind? 


620.  Wind  is  air  put  in  motion.     The  air  is 
never  entirely  free  from  motion,  but  the  ve- 
locity with  which  it  moves  is  perpetually  varying. 

621.  The  principal  cause  of  movements  in 

What    is    the         ,  i_  •        i  '       • 

principal  cause    tne  atmosphere  is  the  variation  ot  temperature 
produced  by  the  alternation  of  day  and  night 
and  the  succession  of  the  seasons. 

"When,  through  the  agency  of  the  sun,  a  particular  portion 
of  the  earth's  surface  is  heated  to  a  greater  degree  than  the 
uce  wind"?         remainder,   the  air  resting  upon  it  becomes  rarefied    and 


How  can  vari- 
ations of  tem- 
perature pro- 
dm 


282  WELLS'S   NATURAL   PHILOSOPHY. 

ascends,  while  a  current  of  cold  air  rushes  in  to  supply  the  vacancy.  Two 
currents,  the  one  of  warm  air  flowing  out,  and  the  other  of  cold  air  flowing 
in,  are  thus  continually  produced ;  and  to  these  movements  of  the  atmosphere 
we  apply  the  designation  of  wind. 

If  the  whole  surface  of  the  earth  were  covered  with  water, 

JrtTj^icai0  fS-      the  winds  would  always  follow  the  sun,  and  blow  uniformly 

tures    of    the     from  east  to  west.     The  direction  of  the  wind  is,  however, 

SiSd8^ffeCtth8      continually  subject  to  interruption  from  mountains,  deserts, 

plains,  oceans,  etc. 

Thus  mountains  which  are  covered  with  snow,  condense  and  cool  the  air 
brought  in  contact  with  them,  and  when  the  temperature  of  the  current  of 
air  constituting  the  wind  is  changed,  its  direction  is  liable  to  be  changed  also. 
The  ocean  is  never  heated  to  the  same  degree  as  the  land,  and  in  conse- 
quence of  this,  the  general  direction  of  the  wind  is  from  tracts  of  ocean  to- 
ward tracts  of  land. 

In  those  parts  of  the  world  which  present  an  extended  surface  of  water, 
the  wind  blows  with  a  great  degree  of  regularity. 

What  is  the  ^22.  Every  variation  exists  in  the  speed  of  winds,  from 
velocity  ^  and  the  mildest  zephyr  to  the  most  violent  hurricane. 

A  wind  which  is  hardly  perceptible  moves  with  a  velocity 
of  about  one  mile  per  hour,  and  with  a  perpendicular  force  on  one  square  foot 
of  -005  pounds  avoirdupois. 

In  a  storm,  the  velocity  of  the  wind  is  from  50  to  60  miles  per  hour,  and 
the  pressure  from  7  to  12  pounds  per  square  foot.  In  some  hurricanes,  the 
velocity  has  been  estimated  at  from  80  to  100  miles  per  hour,  with  a  varying 
force  of  from  30  to  50  pounds. 

The  force  of  the  wind  is  ascertained  by  ob- 

How     is     the  ,  «  .          .       ^ 

force  of  wind    serving  the  amount  oi  pressure  that  it  exerts 

calculated?  .  ,  „  ,.       , 

upon  a  given  plane  surface  perpendicular  to  its 
own  direction. 

If  the  pressure-plate  acts  freely  upon  spiral  springs,  the  power  of  the  wind 
is  denoted  by  the  extent  of  their  compression,  which  thus  becomes  a  measure 
of  their  force,  the  same  as  in  weighing  by  the  ordinary  spring-balance. 

what  is  an  An  instrument  for  measuring  the  force  of 

Anemometer?  foe  wind  is  called  an  Anemometer.  ,,, 

HOW  may  winds  623.    Winds   may  be    divided   into   three 

be  divided?  classes  :_Constant/ Periodical,  and  Variable 
winds. 

624.  In  many  parts  of  the  Atlantic  and  Pacific  oceans,  the 
What  are  the  .  ,  ,  .  . "?,  r  .„  ., 

trade-winds  ?      wind  blows  with  a  uniform  force  and  constancy,  so  that  a  ves- 
sel may  sail  for  weeks  without  altering  the  position  of  a  sail 
or  spar.     Such  winds  have  received  the  designation  of  trade-winds,  inasmuch 
as  they  are  most  convenient  for  navigation,  and  always  blow  in  one  direction. 


WINDS.  283 

w        .  The  trade-winds  are  caused  by  the  movements  of  vast  cur- 

cause  of  the  rents  of  air  which  are  continually  flowing  between  the  poles 
trade-winds  ?  and  tjie  6qUatOr.  Thus  the  air  which  has  been  greatly  heated 
by  the  sun  in  regions  near  to  the  equator,  rises  and  runs  over  toward  either 
pole  in  two  grand  upper  currents,  under  which  there  flows  from  north  and 
south  two  other  currents  of  colder  air  to  occupy  the  space  vacated,  and  to  re- 
store the  equilibrium. 

625.  In  the  northern  hemisphere  the  trade-winds  blow  from 
the  direction  of  the  north-east,  and  hi  the  southern  hemisphere  from  the  south- 
the  trade-winds  ?  eag^ 

The  reason  they  do  not  blow  from  the  direct  north  and  south  is  owing  to 
the  revolution  of  the  earth.  The  circumference  of  the  earth  being  larger  at 
the  equator  than  at  the  poles,  every  spot  of  the  equatorial  surface  must  move 
much  faster  than  the  corresponding  one  at  the  poles :  when,  therefore,  a  cur- 
rent of  air  from  the  poles  flows  toward  the  equator,  it  comes  to  a  part  of  the 
earth's  surface  which  is  moving  faster  than  itself;  in  consequence  of  which 
it  is  left  behind,  and  thus  produces  the  effect  of  a  current  moving  in  the  op- 
posite direction. 

The  region  over  which  the  trade- winds  prevail  extends  for  about  25  degrees 
of  latitude,  on  each  side  of  the  equator,  in  the  Atlantic  and  Pacific  oceans. 

The  reason  the  trade- winds  do  not  blow  uninterruptedly  from  the  equator 
to  each  pole  is  owing  to  the  change  which  takes  place  in  their  temperature  as 
they  move  north  and  south.  Thus  in  the  northern  hemisphere  the  hot  air 
that  ascends  from  the  equator  and  passes  north,  gradually  cools,  and  becomes 
denser  and  heavier,  running  as  it  does  over  the  cold  current  below.  The 
cold  air  from  the  pole,  too,  gradually  becomes  warmer  and  lighter  as  it  passes 
Bouth,  so  that  in  the  temperate  climates  there  is  a  constant  struggle  as  to 
which  shall  have  the  upper  and  which  the  lower  position.  In  these  regions, 
consequently,  there  are  no  uniform  winds.* 

what  are  mon-       626.  Monsoons  are  periodical  currents  of  air 
wons?        which  in  the  Arabian,  Indian,  and  China  seas 
blow  for  nearly  six  months  of  the  year  in  one  direction, 
and  for  the  other  six  in  a  contrary  direction. 

They  are  called  monsoons  from  an  Arabic  word  signifying  season ;  they  are 
also  called  periodical  winds,  to  distinguish  them  from  the  trade-winds  which 
are  constant. 

What  is  the  -  The  tbeory  of  tne  monsoons  ja  as  follows : — During  six 
theory  of  the  months  of  the  year,  from  April  to  October,  the  air  of  Arabia, 
monsoons?  Persia,  India,  and  China,  is  so  rarefied  by  the  enormous  heat 

of  their  summer  sun,  that  the  cold  air  from  the  south  rushes  toward  these 

*  The  existence  of  a  great  current  of  air  in  the  upper  regions  of  the  atmosphere,  flow- 
ing in  an  nearly  contrary  direction  to  the  trade-winds,  has  been  confirmed  by  the  ob- 
servations of  travelers  who  have  ascended  the  Peak  of  Teneriffe,  or  some  of  the  high 
mountains  in  the  islands  of  the  Southern  Pacific  Ocean.  At  a  height  of  about  12,000  feet 
a  wind  is  encountered,  blowing  constantly  in  an  opposite  direction  to  that  which  prevails 
nt  the  level  of  the  sea  below. 


284  WELLS'S   NATURAL   PHILOSOPHY. 

countries,  across  the  equator,  and  produces  a  south-west  wind.  When  the 
sun,  on  the  other  hand,  has  left  the  northern  side  of  the  equator  for  the 
southern,  the  southern  hemisphere  is  rendered  hotter  than  the  northern,  and 
the  direction  of  the  wind  is  reversed,  or  the  monsoon  blows  north-east  from 
October  to  April 

The  monsoons  are  more  powerful  than  the  trade-winds,  and  very  often 
amount  to  violent  gales.  They  are  also  more  useful  than  the  trade-winds, 
since  the  mariner  is  able  to  avail  himself  of  their  periodic  changes  to  go  in 
one  direction  during  one  half  of  the  year,  and  return  hi  the  opposite  direction 
during  the  other  half. 

What   is    th  ^^'  ^n  some  Par^s  of  the  world,  as  on  coasts  and  islands, 

explanation  of  the  heating  action  of  the  sun  produces  daily  periodical  winds, 
breezes"4  Se*  wn^c^  are  termed  land  and  sea-breezes. 

During  the  day,  the  land  becomes  much  more  highly  heated 
by  the  sun  than  the  adjacent  water,  and  consequently  the  air  resting  upon 
the  land  is  much  more  heated  and  rarefied  than  that  upon  the  water.  The 
cooler  and  denser  air,  therefore,  flows  from  the  water  toward  the  land,  con- 
stituting a  sea-breeze,  and,  displacing  the  warmer  and  lighter  air  over  the 
land,  forces  it  into  a  higher  region,  along  which  it  flows  in  an  upper  current 
seaward. 

At  night  a  contrary  effect  is  produced.  After  sunset  the  land  cools  much 
more  rapidly  than  the  water,  and  the  air  over  the  shore  becoming  cooler, 
and  consequently  heavier  than  that  over  the  sea,  flows  toward  the  water  and 
forms  the  land-breeze.* 

The  phenomena  of  land  and  sea-breezes  may  be  well  illustrated  by  a  simple 
experiment  Fill  a  large  dish  with  cold  water,  and  place  in  the  middle  of  it 
a  saucer  full  of  warm  water ;  let  the  dish  represent  the  ocean,  and  the  saucer 
an  island  heated  by  the  sun,  and  rarefying  the  air  above  it ;  blow  out  a  can- 
dle, and  if  the  air  of  the  room  be  still,  on  applying  it  successively  to  every 
side  of  the  saucer,  the  smoke  will  be  seen  moving  toward  it  and  rising  over 
it,  thus  indicating  the  course  of  the  air  from  sea  to  land.  On  reversing  the 
experiment,  by  filling  the  saucer  with  cold  water,  and  the  dish  with  warm, 
the  land-breeze  will  be  shown  by  holding  the  smoking  wick  over  the  edge 
of  the  saucer ;  the  smoke  will  then  be  wafted  to  the  warmer  air  over  the  dish. 

628.  In  the  temperate  zones,  the  winds  have 
do      variable    little  of  regularity,   and  these  latitudes   are 

winds  prevail?  °  .  ' '  . 

known  as  the  regions  of  "  variable  winds. 

In  the  tropics,  the  great  aerial  currents  known  as  the  trade-winds  exist  in 
all  their  power,  and  control  most  of  the  local  influences ;  but  in  the  temperate 
zones,  where  the  force  of  the  trade-winds  is  diminished,  a  perpetual  contest 

*  Advantage  is  taken  of  these  breezes  by  coasters,  which,  drawing  less  water  than 
larger  vessels,  can  approach  the  coast  within  those  limits  where  the  sea  and  land-breezes 
first  begin  to  operate.  Thus  a  ship  of  war  may  not  be  able  to  take  advantage  of  these 
winds,  while  sloops  and  schooners  may  be  moving  along  close  to  the  shore  under  a  press 
of  canvas,  and  be  out  of  sight  before  the  larger  vessel  is  released  from  the  calm  bordering 
these  breezes,  and  fringing  for  some  time  the  beach  only. 


WINDS.  285 

occurs  between  the  permanent  and  temporary  currents,  giving  rise  to  con- 
stant fluctuations  in  the  strength  and  direction  of  the  winds. 

629.  The  driest  winds  of  the  United  States  are  west  and 
character     of     north-west  winds,  since  they  blow  over  great  tracts  of  land, 
the  winds  of     and  have  little  opportunity  of  absorbing  moisture. 
States?    D  The  south  winds  are  generally  warm  and  productive  of 

rain,  since  coming  from  tropical  countries,  they  are  highly 
heated,  and  readily  absorb  moisture  as  they  pass  over  the  ocean.  As  soon, 
however,  as  they  reach  a  cold  climate  they  are  condensed,  and  can  no  longer 
hold  all  their  vapor  in  suspension ;  hi  consequence  of  which  some  of  it  is  de- 
posited as  rain. 

630.  Other  disturbances  of  the  air  occasion  a  variety  of  phenomena  known 
as  "Simoons,"  "Hurricanes,"  "Tornadoes,"  "Water-Spouts,"  etc. 

what  is  a  si-  631.  The  Simoon  is  an  intensely  hot  wind 
that  prevails  upon  the  vast  deserts  of  Africa 
and  the  arid  plains  of  Asia,  causing  great  suffering,  and 
often  destruction  of  whole  caravans  of  men  and  animals 
when  encountered.  Its  origin  is  to  be  sought  in  the  pecu- 
liarities of  the  soil  and  the  geographical  position  of  the 
countries  where  it  occurs. 

"  The  surface  of  the  deserts  of  Africa  and  Asia  is  composed  of  dry  sand, 
which  the  vertical  rays  of  the  sun  render  burning  to  the  touch.  The  heat  of 
these  regions  is  insupportable,  and  their  atmosphere  like  the  breath  of  a  fur- 
nace. When,  under  such  circumstances,  the  wind  rises  and  sweeps  over 
these  plains,  it  is  intensely  hot  and  destitute  of  moisture,  and  at  the  same  tune 
bears  aloft  with  it  great  clouds  of  fine  sand  and  dust — a  dreadful  visitant  to 
the  traveler  of  the  desert." 

The  Hurricane  is  a  remarkable  storm  wind, 
peculiar  to  certain  portions  of  the  world.  It 
rarely  takes  its  rise  beyond  the  tropics,  and  it  is  the  only 
storm  to  dread  within  the  region  of  the  trade-winds. 

Hurricanes  are  especially  distinguished  from  all  other  kinds  of  tempests  by 
their  extent,  irresistible  power,  and  the  sudden  changes  that  occur  in  the 
direction  of  the  wind. 

In  the  northern  hemisphere,  the  hurricane 

At  what  times  .  ,  . 

and  locations    most  frequently  occurs  in  the  regions  of  the 

do  hurricanes       ,_TT  Ti«  •         t  t  t  •       i 

most  frequent-  West  Indies  ;  in  the  southern  hemisphere,  it 
occurs  in  the  neighborhood  of  the  Island  of 
Mauritius,  in  the  Indian  Ocean.  They  also  seem  to  be 
confined  to  particular  seasons  ;  thus  the  West  Indian 
occur  from  August  to  October  ;  the  Mauritian  from  Feb- 
ruary to  April. 


286  WELLS'S  NATURAL  PHILOSOPHY. 

Kecent  investigations  have  proved  the  hur- 

What    is    the  .     c  r 

nature  of  the     iicanes  to  consist  or  extensive  storms  of  wind, 

hurricane?  ,  .    ,  ,  ,  ...  .     . 

which  revolve  round  an  axis  either  upright  or 
inclined  to  the  horizon  ;  while  at  the  same  time  the  body 
of  the  storm  has  a  progressive  motion  over  the  surface  of 
the  ocean. 

Thus  it  is  the  nature  of  a  hurricane  to  travel  round  and  round  as  well  aa 
forward,  much  as  a  corkscrew  travels  through  a  cork,  only  the  circles  are  all 
flat,  and  described  by  a  rotary  wind  upon  the  surface  of  the  water.  A  ship 
revolving  in  the  circles  of  a  hurricane,  would  find,  in  successive  positions,  the 
wind  blowing  from  every  point  of  the  compass.* 

The  effect  produced  by  a  hurricane  upon  the  atmosphere  is  very  singular. 
As  it  consists  of  a  body  of  air  rotating  in  a  vast  circle,  its  center  is  the  point 
of  least  motion.  Mariners  who  have  been  caught  in  such  a  center,  describe  the 
unnatural  calm  that  prevails  as  awful  —  an  apparent  lull  of  the  tempest,  which 
seems  to  have  rested  only  to  gather  strength  for  greater  efforts.  The  mass 
of  air,  however,  which  constitutes  the  body  of  the  storm  will  be  driven  out- 
ward from  the  center  toward  the  margin,  just  as  water  in  a  pail  which  is 
made  to  revolve  rapidly  flies  from  the  center  and  swells  up  the  sides.  But 
the  pressure  of  the  atmosphere  beyond  the  whirl,  checking  and  resisting  the 
centrifugal  force,  at  length  arrests  the  outward  progress  of  the  mass  of  air, 
and  limits  the  storm. 

The  progressive  velocity  of  hurricanes  is  from  seventeen  to 
respecting  the  forty  miles  per  hour  ;  but  distinct  from  the  progressive  velocity 
^aoes^tra  "nf  *s  *^ie  r°tary  velocity,  which  increases  from  the  exterior  bound- 
ed by  hurri-  ary  to  the  center  of  the  storm,  near  which  point  the  force  of 
? 


the  tempest  is  greatest,  the  wind  sometimes  blowing  at  the 
rate  of  one  hundred  miles  per  hour. 

The  distance  traversed  by  these  terrible  tempests  is  also  immense.  The 
great  gale  of  August,  1830,  which  occurred  at  St.  Thomas,  in  the  West 
Indies,  on  the  12th,  reached  the  Banks  of  Newfoundland  on  the  19th,  having 
traveled  more  than  three  thousand  nautical  miles  in  seven  days  ;  the  track  of 
the  Cuba  hurricane  of  1844  was  but  little  inferior  hi  length. 

The  surface  simultaneously  swept  by  these  tremendous  whirlwinds  is  a 
vast  circle  varying  from  one  hundred  to  five  hundred  miles  in  diameter. 

Mr.  Redfield  has  estimated  the  great  Cuba  hurricane  of  1844  to  have  been 
not  less  than  eight  hundred  miles  in  breadth,  and  the  area  over  which  it  pre- 
vailed during  its  whole  length  was  computed  to  be  two  million  four  hun- 
dred thousand  square  miles  —  an  extent  of  surface  equal  to  two  thirds  of 
that  of  all  Europe. 

*  In  1845,  a  ship  encountered  a  hurricane  near  Mauritius.  The  wind,  as  the  ship 
sailed  in  the  circuit  of  the  storm,  changed  five  times  completely  round  in  one  hundred 
and  seventeen  hours.  The  whole  distance  sailed  by  the  vessel  was  thirteen  hundred  and 
seventy-three  miles,  and  at  the  termination  of  the  storm  she  was  only  three  hundred  and 
fifty-four  miles  from  the  placo  where  the  storm  commenced. 


WINDS.  287. 

what  are  Tor-         632.  Tornadoes  may  be  regarded  as  hurri- 
nadoes?        canes,  differing  chiefly  in  respect  to  their  con- 
tinuance and  extent. 

Tornadoes  usually  last  from  fifteen  to  seventy  seconds  ; 
their  breadth  varies  from  a  few  rods  to  several  hundred 
yards,  and  the  length  of  their  course  rarely  exceeds  twenty 
miles. 

The  tornado  is  generally  preceded  by  a  calm  and  sultry  state  of  the  atmos- 
phere, when  suddenly  the  whirlwind  appears,  prostrating  every  thing  before 
it.  Tornadoes  are  usually  accompanied  with  thunder  and  lightning,  and 
sometimes  showers  of  hail. 

Tornadoes  are  supposed  to  be  generally  pro- 

IIow  are  tor-        ,  11,11.1  ,•  n 

nadoes     pro-    duced  by  the  lateral  action  of  an  opposing 
wind,  or  the  influence  of  a  brisk  gale  upon  a 
portion  of  the  atmosphere  in  repose. 

Similar  phenomena  are  seen  in  the  eddies,  or  little  whirlpools  formed  in 
water,  when  two  streams  flowing  in  different  directions  meet.  They  occur 
most  frequently  at  the  junction  of  two  brooks  or  rivers. 

Whirlwinds  on  a  small  scale  are  often  produced  at  the  corners  of  streets  in 
cities,  and  are  occasioned  by  a  gust  of  wind  sweeping  round  a  building,  and 
striking  the  calm  air  beyond. 

The  whirl  of  a  tornado,  or  whirlwind,  appears  to  originate  in  the  higher 
regions  of  the  atmosphere ;  it  increases  in  velocity  as  it  descends,  its  base 
gradually  approaching  the  earth,  until  it  rests  upon  the  surface. 

Great  conflagrations  sometimes  produce  whirlwinds,  in  consequence  of  a 
strong  upward  current,  which  is  produced  by  the  expansion  of  the  heated 
air.  A  remarkable  example  of  this  is  recorded  to  have  happened  at  the 
burning  of  Moscow,  in  1812,  where  the  air  became  so  rarefied  by  heat,  that 
the  wind  rose  to  a  frightful  hurricane. 

It  has  been  noticed  as  one  of  the  curious  effects  of  a  tornado,  that  fowls  and 
birds  overtaken  by  it  and  caught  in  its  center,  are  often  entirely  stripped  of 
their  feathers.  In  a  theory  propounded  some  years  since  to  the  American 
Association  for  the  Promotion  of  Science,  by  an  eminent  scientific  authority, 
it  was  supposed  that  in  the  vortex,  or  center  of  the  tornado,  there  was  a 
vacuum,  and  the  fowls  being  suddenly  caught  in  it,  the  air  contained  in  the 
barrel  of  their  quills  expanded  with  such  force  as  to  strip  them  from  the 
body. 

what    is    a        6%3-  A  water-spout  is  a  whirlwind  over  the 

Water-spout?        gurface  Qf  watCTj  and    differg  from  ft  whirlwind 

on  land  in  the  fact  that  water  is  subjected  to  the  action 
of  the  wind,  instead  of  objects  on  the  surface  of  the  earth. 
In  diameter  the  spout  at  the  base  ranges  from  a  few  feet; 


288 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  228. 


to  several  hundreds,  and  its  altitude  is  supposed  to  be 
often  upward  of  a  mile. 

When  an  observer  is  near  to  the  spout,  a  loud  hissing  noise  is  heard,  and 
the  interior  of  the  column  seems  to  be  traversed  by  a  rushing  stream. 

The  successive  appearances  of  a  water- 
spout are  as  follows : — At  first  it  appears  to 
be  a  dark  cone,  extending  from  the  clouds 
to  the  water;  then  it  becomes  a  column 
uniting  with  the  water.  After  continuing 
for  a  little  time,  the  column  becomes  dis- 
united, the  cone  reappears,  and  is  gradually 
drawn  up  into  the  clouds.  These  various 
changes  are  represented  in  Fig.  228.  It  is 
a  common  belief  that  water  is  sucked  up  by 
the  action  of  the  spout  into  the  clouds ;  but 
there  is  reason  to  suppose  that  water  rather 
descends  from  the  clouds,  as  water  which 
has  fallen  from  a  spout  upon  the  deck  of  a 
vessel  has  been  found  to  be  fresh.  There  ia 
no  evidence,  furthermore,  that  a  continuous 
column  of  water  exists  within  the  whirling  pillar. 


What    are 
Meteorites  ? 


SECTION    IT. 

METEORIC     PHENOMENA. 

634.  Meteorites  are  luminous  bodies,  which 
from  time  to  time  appear  in  the  atmosphere, 

moving  with  immense  velocity,  and  remaining  visible  but 
for  a  few  moments.  They  are  generally  accompanied  by 
a  luminous  train,  and  during  their  progress  explosions  are 
often  heard. 

635.  The  term  aerolite  is  given  to  those 
stony  masses  of  matter  which  are  sometimes 

seen  to  fall  from  the  atmosphere.* 

What  is  known         The  weight  of  those  aerolites  which  have  been  known  to 

respecting  the     fan  from  the  atmosphere  varies  from  a  few  ounces  to  several 

•weight  and  ve-      ,        ,      ,  , 

locity  of  aero-     hundred  pounds,  or  even  tons. 

lites  ?  The  height  above  the  earth's  surface  at  which  they  are  sup- 

posed to  make  their  appearance  has  been  estimated  to  vary  from  18  to  80  miles. 

*  Aerolite  is  derived  from  the  Greek  words  acp  (atmosphere)  and  X«0o$  (a  stone).  A 
meteor  is  distinguished  from  an  aerolite  by  the  fact  that  it  bursts  in  the  atmosphere,  but 
leaves  no  residuum,  while  the  aerolite,  which  is  supposed  to  be  a  fragment  of  a  meteor, 
comes  to  the  ground. 


What    are 
Aerolites  ? 


METEORIC   PHENOMENA.  289 

The  estimated  velocity  of  these  bodies  is  somewhat  more  than  three  hun- 
dred miles  per  minute,  though  one  meteor  of  immense  size,  which  is  supposed 
to  have  passed  within  twenty-five  miles  of  the  earth,  moved  at  the  rate  of 
twelve  hundred  miles  per  minute.  Owing,  however,  to  the  short  time  the 
meteor  is  visible,  and  its  great  velocity,  accurate  observations  can  not  be 
made  upon  it ;  and  all  estimates  respecting  their  distance,  size,  etc.,  must  be 
considered  as  only  approximations  to  the  truth. 

Very  many  of  the  meteorites  which  have  fallen  at  different 
respecting  the  times  and  in  different  parts  of  the  globe,  resemble  each  other 
constitution  of  so  cioseiy  that  they  would  seem  to  have  been  broken  from 

aerolites  ?  J 

the  same  piece  or  mass  of  matter. 

Most  of  them  are  covered  with  a  black  shining  crust,  as  if  the  body  had 
been  coated  with  pitch.  When  broken,  their  color  is  ash-gray,  inclining  to 
black.  They  consist  for  the  most  part  of  malleable  iron  and  nickel,  but  they 
often  contain  small  quantities  of  other  substances.  They  do  not  resemble  in 
composition  any  other  bodies  found  upon  the  surface  of  the  earth,  but  have  a 
character  of  their  own  so  peculiar  that  it  enables  us  to  decide  upon  the  me- 
teoric origin  of  masses  of  iron  which  are  occasionally  found  scattered  up  and 
down  the  surface  of  the  earth,  as  in  the  south  of  Africa,  in  Mexico,  Siberia, 
and  on  the  route  overland  to  California.  Some  of  these  masses  are  of  immense 
weight,  and  undoubtedly  fell  from  the  atmosphere. 

what  is  the  636.  Four  hypotheses  have  been  advanced 
ofpp°meteoricn  to  account  for  the  origin  of  these  extraordinary 
bodies?  bodies  :  1.  That  they  are  thrown  up  from  ter- 

restrial volcanoes  ;  2.  That  they  are  produced  in  the  at- 
mosphere from  vapors  and  gases  exhaled  from  the  earth; 
3.  That  they  are  thrown  from  lunar  volcanoes  ;  4.  That 
they  are  of  the  same  nature  as  the  planets,  either  derived 
from  them,  or  existing  independently. 

The  fourth  of  these  suppositions  most  fully  explains  the  facts  connected 
with  the  appearance  of  meteorites,  and  the  third  likewise  has  some  strong 
evidence  in  its  favor. 

HOW  do  shoot-  ®37.  Shooting-stars  differ  in  many  respects 
JSfm^^w  ?  from  meteors.  Their  altitude  and  velocity  are 
greater  ;  they  are  far  more  numerous  and  fre- 
quent, and  are  unaccompanied  by  any  sound  or  explosion. 
Their  brilliancy  is  also  much  inferior  to  that  of  the  me- 
teor, and  no  portion  of  their  substance  is  ever  known  to 
have  reached  the  earth. 

At  what  height  Tne  altitude  °^  shootmg-stars  is  supposed  to  vary  from  six 
do  shooting-  to  four  hundred  and  sixty  miles,  the  greatest  number  appear- 
stars  appear?  ^  ^  ft  heig^  Of  ab0ut  seventy  miles.  Owing  to  their  mun» 

13 


290  WELLS'S   NATURAL   PHILOSOPHY. 

ber  and  frequency  of  occurrence,  many  careful  observations  have  been  mado 
upon  them,  with  a  view  of  determining  these  facts. 

Their  velocity  is  supposed  to  range  from  sixty  to  fifteen  hundred  miles  per 
minute. 

Some  of  these  meteoric  appearances  may  be  seen  every  clear  night,  but 
they  appear  to  fall  in  great  numbers  at  certain  periodical  epochs.  The  pe- 
riods when  they  may  be  noticed  most  abundantly  are  on  the  9th  and  10th  of 
August,  and  the  12th  and  13th  of  November.* 

The  majority  of  shooting  stars  appear  to  radiate  from 
a  particular  part  of  the  heavens,  viz.,  a  point  in  the  con- 
stellation Perseus,  undoubtedly  far  beyond  the  limits  of  our 
atmosphere. 

What  theories  In  OT^eT  to  account  for  the  °"gin  of  shooting  stars,  it  has 
have  been  pro-  been  supposed  by  Prof.  Olmstead,  that  they  are  derived  from 
count  for  the  a  bodv  comPosed  of  matter  exceedingly  rare,  like  the  tail  of  a 
origin  of  shoot-  comet,  revolving  around  the  sun  within  the  orbit  of  the  earth, 
ing  stars  ?  -Q  a  Space  little  less  than  a  year ;  and  that  at  times  the  body 

approaches  so  near  the  earth  that  the  extreme  portions  become  detached  and 
drawn  to  the  earth  by  virtue  of  its  great  attraction.  It  has  been  further  sup- 
posed that  the  matter  of  which  these  bodies  is  composed  is  combustible,  and 
becomes  ignited  on  entering  the  earth's  atmosphere. 

The  nearest  approach  of  the  central  body  to  the  earth  is  supposed  to  bo 
about  2,000  miles.  Bodies  falling  from  this  distance  would  enter  the  earth's 
atmosphere  at  a  height  of  at  least  50  miles  above  the  surface,  with  a  velocity 
generated  by  the  force  of  gravity  above  4  miles  per  second — a  velocity  teu 
times  greater  than  the  utmost  speed  of  a  cannon-ball. 

When  common  air  is  compressed  in  a  tight  cylinder  to  the  extent  of  one 
fifth  of  its  volume,  sufficient  heat  is  generated  to  ignite  tinder.  If  we  suppose 
that  the  fragments  descend  with  such  velocity  as  to  compress  the  rarefied 
atmosphere  at  the  height  of  30  miles  to  such  an  extent  only  as  to  make  it  aa 
dense  as  ordinary  air,  the  temperature  would  be  raised  as  high  as  46,000°  F. 
— a  heat  far  more  intense  than  can  be  generated  in  any  furnace.  Unless, 
therefore,  the  mass  of  matter  comprising  the  body  was  very  large,  it  must  bo 
dissipated  by  heat  long  before  it  reaches  the  surface  of  the  earth. 

Another  theory  has  been  proposed  by  the  eminent  astronomer  Chaldini, 
•who  supposes  that,  in  addition  to  the  planets  and  their  satellites  which  revolve 
about  the  sun,  there  are  innumerable  smaller  bodies ;  and  that  these  occa- 
sionally enter  within  the  atmosphere  of  the  earth,  take  fire,  or  descend  to  its 
surface. 

•  They  hare  also  been  noticed  in  unusual  abundance  on  the  18th  of  October,  the  6th 
and  7th  of  December,  the  2d  of  January,  the  23d  and  24th  of  April,  and  from  the  18th  to 
the  20th  of  June. 

Four  most  remarkable  meteoric  showers  have  been  noticed,  viz.,  in  1797, 1831, 1832,  and 
1833,  all  in  the  month  of  November.  In  the  shower  of  1833,  the  meteors,  in  many  parts  of 
ihe  United  States,  appeared  to  fall  as  thick  as  snow-flakes. 


POPULAR  OPINIONS   CONCERNING  THE  WEATHER.    291 
SECTION    V. 

POPULAR     OPINIONS     CONCERNING     TKE     "WEATHER. 

638.  There  is  no  reason  to  doubt  that  every 
th°c  aw!Sther  change  in  the  weather  is  in  strict  accordance 
cordance  with  with  some  definite  physical  agencies,  which  are 
fixed  and  certain  in  their  operations.  We 
can  not,  however,  foretell  with  any  degree  of  certainty 
the  character  of  the  weather  for  any  particular  time,  be- 
cause the  laws  which  govern  meteorological  changes  are 
as  yet  imperfectly  understood. 

There  are,  however,  in  all  countries,  certain  ideas  and  pop- 
Are  the  popu-  '  , ,  ,          ,       .   a 
lar  ideas   re-     ular  proverbs  respecting  changes  in  the  weather,  the  mflu- 

specting  ence  Qf  faQ  mool,  the  aurora  borealis,  etc.,  which  are  wholly 

changes  in  the  . 

weather  found-     erroneous  and  unworthy  of  belief;    since,  when  tested  by 
ed  on  fact?  long-continued  observations,  they  are  invariably  found  to  be 

unsupported  by  evidence. 

Thus  an  examination  of  meteorological  records,  kept  in  different  countries, 
through  many  years,  proves  conclusively  that  the  popular  notions  concerning 
the  influence  of  the  moon  on  the  weather  has  no  foundation  in  any  well- 
established  theory,  and  no  correspondence  with  observed  facts. 

There  is,  however,  some  reason  for  supposing  that  rain  falls  more  frequently 
about  four  days  before  full  moon,  and  less  frequently  about  four  or  five  days 
before  new  moon,  than  at  other  parts  of  the  month;  but  this  can  not  be  con- 
sidered as  an  established  fact.  In  other  respects,  the  changes  of  the  moon 
can  not  be  shown  to  have  influenced  in  any  way  the  production  of  rain. 

There  is  also  a  current  belief  among  many  persons  that  timber  should  be 
cut  during  the  decline  of  the  moon.  To  test  the  matter,  an  experiment,  on 
an  extensive  scale,  was  made  some  years  since  in  France,  when  it  was  found 
that  there  was  no  difference  in  the  quality  of  any  timber  felled  in  different 
parts  of  the  lunar  month. 

It  is  also  supposed  that  bright  moonlight  hastens,  in  some  way,  the  putre- 
faction of  animal  and  vegetable  substances.  The  facts  in  respect  to  this  sup- 
position are,  that  on  bright,  clear  nights,  when  the  moon  shines  brilliantly, 
dew  is  more  freely  deposited  on  these  substances  than  at  other  times,  and  in 
this  way  putrefaction  may  be  accelerated.  With  this  result  the  moon  has  no 
connection. 

It  is  a  traditional  idea  with  many  that  a  long  and  violent  storm  usually 
accompanies  the  period  of  the  equinoxes,  especially  the  autumnal ;  but  the 
examination  of  weather  records  for  sixty-four  years  has  shown  that  no 
particular  day  can  be  pointed  out  in  the  month  of  September  (when  the 
"  equinoctial  storm"  is  said  to  occur)  upon  which  there  ever  was,  or  ever  will 


292  WELLS'S   NATURAL   PHILOSOPHY. 

be,  a  so-called  equinoctial  storm.  The  fact,  however,  should  Hot  be  concealed, 
that,  taking  the  average  for  the  five  days  embracing  the  equinox  for  the 
period  above  stated,  the  amount  of  rain  is  greater  than  for  any  other  five 
days,  by  three  per  cent.,  throughout  the  month. 

Observations  recorded  for  a  long  period  have  proved  that  the  phenomenon 
of  the  aurora  borealis,  which  is  said  to  precede  a  storm,  is  as  often  followed 
by  fair,  as  by  foul  weather. 

Meteorological  records,  kept  for  eighty  years  at  the  observatory  of  Green- 
wich, England,  seem  to  show  that  groups  of  warm  years  alternate  with  cold 
ones  in  such  a  way  as  to  render  it  probable  that  the  mean  annual  tempera- 
tures rise  and  fall  in  a  series  of  curves,  corresponding  to  periods  of  about  four- 
teen years. 

There  is  little  doubt  that  some  animals  and  insects  are  able  to  foretell 
changes  in  the  weather,  when  man  fails  to  perceive  any  indications  of  the 
same.  Thus  some  varieties  of  the  land-snail  only  make  their  appearance  be- 
fore a  rain.  Some  other  varieties  of  land  crustaceous  animals  change  their 
color  and  appearance  twenty-four  hours  before  a  rain. 

For  a  light,  short  rain,  some  trees  have  been  observed  to  incline  their  leaves, 
so  as  to  retain  water ;  but  for  a  long  rain,  they  are  so  arranged  as  to  conduct 
the  water  away. 

The  admonition  given  several  thousand  years  ago,  is  equally  sound  in  its 
philosophy  at  the  present  day :  "  He  that  observeth  the  winds  shall  not  sow ; 
and  he  that  regardeth  the  clouds  shall  not  reap." — Eccks.,  xi.  4. 


CHAPTER    XIV. 

LIGHT. 

639.  LIGHT  is  the  physical  agent  which  oc- 
casions, by  its  action  upon  the  eye,  the  sensa- 
tion of  vision. 

what  is  the        640.  Optics  is  the  name  given  to  that  de- 

tlcsT6  °f  °p"    partment  of  physical  science  which  treats  of 

vision,  and  of  the  laws  and  properties  of  light.* 

Between  the  eye  and  any  visible  object  a  space  of  greater  or  less  extent 
intervenes.  In  some  instances,  as  when  we  look  at  a  star,  the  extent  of  the 
space  existing  between  the  eye  and  the  object  seen  is  so  great,  that  the  mind 
is  unable  to  form  any  adequate  conception  of  it.  Yet  we  recognize  the  ex- 
istence of  objects  at  such  distances,  by  the  physical  effect  which  they  produce 
on  our  organs  of  vision. 

•  From  the  Greek  word  "  Orrro/iaf,"  to  see. 


LIGHT.  293 

641.  In  order  to  explain  how  such  a  result  is  possible,  or 
What  theories  .  „  •,    ,  • 

of  light  have  in  other  words,  to  account  for  the  origin  of  light,  two  theories 
been  proposed?  ^are  been  proposed)  which  are  called  the  CORPUSCULAR  and 
the  UNDULATORY  Theories. 

what  is  the  The  CORPUSCULAR  THEORY  supposes  that  a 
Theo™™1"  distant  object  becomes  visible  to  us  by  emit- 
Light?  tmg  particles  of  matter  from  its  surface,  which 
particles  of  matter,  passing  through  the  intervening  space 
between  the  visible  object  and  the  eye,  enter  the  eye,  and 
striking  upon  the  nervous  membrane,  so  affect  it  as  to 
produce  the  sensation  of  light,  or  vision. 

According  to  this  theory,  there  is  a  striking  analogy  or  resemblance  be- 
tween the  eye  and  the  organs  of  smelling.  Thus,  we  recognize  the  odor  of 
an  object  in  consequence  of  the  material  particles  which  pass  from  the  object 
to  the  organs  of  smelling,  and  there  produce  a  sensation.  In  the  same 
manner,  a  visible  object  at  any  distance  may  be  supposed  to  send  forth  parti- 
cles of  light,  which  move  to  the  eye  and  produce  vision,  by  acting  mechan- 
ically on  its  nervous  structure,  as  the  odoriferous  particles  of  a  rose  produce  a 
sensible  effect  upon  the  organs  of  smelling. 

What    is    the  The    UNDULATORY    THEORY    SUppOSCS    that 

there  exists  throughout  all  space  an  ethereal, 
elastic  fluid,  which,  like  the  air,  is  capable  of 
receiving  and  transmitting  undulations,  or  vibrations. 
These,  reaching  the  eye,  affect  the  optic  nerve,  and  pro- 
duce the  sensation  which  we  call  light. 

According  to  this  theory,  there  is  a  striking  analogy  between  the  eye  and 
the  ear ;  the  vibrations,  or  undulations  of  the  ethereal  medium  being  supposed 
to  pass  along  the  space  intervening  between  the  visible  object  and  the  eye  in 
the  same  manner  that  the  undulations  of  the  air,  produced  by  a  sounding  body, 
pass  through  the  air  between  it  and  the  ear. 

Which  of  the  The  Corpuscular  Theory  was  sustained  by  Newton,  and  was 
two  theories  of  for  a  long  time  generally  believed.  At  the  present  day  it  is 
aUyVe'cefyed  ?"  almost  entirely  discarded,  and  the  Undulatory  Theory  is  now 
received  by  scientific  men  as  substantially  correct ;  since  it 
explains  in  a  satisfactory  manner  nearly  all  the  phenomena  of  light,  which 
the  Corpuscular  Theory  does  not. 

If  the  Corpuscular  Theory  be  correct,  a  common  candle  is  able  to  fill  for 
hours,  with  particles  of  luminous  matter,  a  circle  four  miles  in  diameter,  since 
it  would  be  visible,  under  favorable  circumstances,  in  every  portion  of  this 
space.  Light,  moreover,  has  no  weight ;  the  largest  possible  quantity  col- 
lected in  one  point  and  thrown  upon  the  most  sensitive  balance,  does  not 
affect  it  in  the  slightest  degree. 


294  WELLS'S   NATURAL  PHILOSOPHY. 

what  are  the        The  chief  sources  of  light  are  the  sun,  the 
ofiieght?°urce8    stars>  fire  or  chemical  action,  electricity,  and 
phosphorescence. 

Under  the  head  of  chemical  action  are  included  all  the  forms  of  artificial 
light  which  are  obtained  by  the  burning  of  bodies.  Examples  of  light  pro- 
duced by  phosphorescence,  as  it  is  called,  are  seen  in  the  glow  of  old  and  de- 
cayed wood,  and  in  the  light  emitted  by  fire-flies  and  some  marine  animals. 

642.  All  bodies  are  either  luminous  or  non-luminous. 
mat  is  a  in.        Luminous  bodies  are  those  which  shine  by 

nous  body?        theif    Qwn     j^  .    ^^    f()r     example?     ag    the 

sun,  the  flame  of  a  candle,  metal  rendered  red  hot,  etc. 

All  solid  bodies,  when  exposed  to  a  sufficient  degree  of  heat,  become  lu- 
minous. It  has  been  recently  proved*  that  all  solids  begin  to  emit  light  at 
the  same  degree  of  heat,  viz.,  977°  of  Fahrenheit's  thermometer.  As  the 
temperature  rises,  the  brilliancy  of  the  light  rapidly  increases,  so  that  at  a 
temperature  of  2600°  it  is  almost  forty  times  as  intense  as  at  1900°.  Gases 
must  be  heated  to  a  much  greater  extent  before  they  begin  to  emit  light. 

what  in  anon-        Non-luminous  bodies  are  those  which  pro- 

nousbody? 


rendered  temporarily  luminous  by  being  placed  in  the 
presence  of  luminous  bodies. 

Thus,  the  sun,  or  a  candle,  renders  objects  in  an  apartment  luminous,  and 
therefore  visible;  but  the  moment  the  sun  or  candle  is  withdrawn,  they  be- 
come invisible. 

what  are  trans-       Transparent  bodies  are  those  which  do  not 
parent  bodies?   mterrupt  the  passage  of  light,  or  which  allow 
other  bodies  to  be  seen  through  them.     Glass,  air,  and 
water  are  examples  of  very  transparent  bodies. 
what      are        Opaque  bodies  are  those  which  do  not  permit 

opaque  bodies?      ]-ght     to    pags     tnrough    them>        The    petals, 

stone,  earth,  wood,  etc.,  are  examples  of  opaque  bodies. 

Transparency  and  opacity  exist  in  different  bodies  in  very  different  degrees, 
We  can  not  clearly  explain  what  there  is  in  the  constitution  of  one  mass  of 
matter,  as  compared  with  another,  which  fits  the  one  to  transmit  light,  and 
the  other  to  obstruct  it  ;  but  the  arrangement  of  the  particles  has  undoubt- 
edly much  influence. 

Strictly  speaking,  there  is  no  body  which  is  perfectly  transparent,  or  per- 

fectly opaque.     Some  light  is  evidently  lost  in  passing  even  through  space, 

and  still  more  in  traversing  our  atmosphere.     It  has  been  calculated  that  the 

atmosphere,  when  the  rays  of  the  sun  pass  perpendicularly  through  it,  inter- 

*  By  Prof.  J.  W.  Draper. 


LIGHT.  295 

cept  from  one  fifth  to  one  fourth  of  their  light:  but  when  the  sun  is  near  the 
horizon,  and  the  mass  of  air  through'which  the  solar  rays  pass  is  consequently 
vastly  increased  in  thickness,  only  1-2 12th  part  of  their  light  can  reach  the 
surface  of  the  earth.  If  our  atmosphere,  in  its  state  of  greatest  density,  could 
be  extended  rather  more  than  700  miles  from  the  earth's  surface,  instead  of 
40  or  50,  as  it  is  at  present,  the  sun's  rays  could  not  penetrate  through  it, 
and  our  globe  would  roll  on  in  darkness.  Bodies,  on  the  contrary,  which 
are  considered  as  perfectly  opaque,  will,  if  made  sufficiently  thin,  allow  light 
to  pass  through  them.  Thus,  gold-leaf  transmits  a  soft,  green  light. 

643.  Light,  from  whatever  source  it  may  be 

In  what  man-  fo  .  .  J.    , 

ner   is   light    derived,  moves,  or  is  propagated   in  straight 

propagated?  J.  fe 

lines,  so  long  as  the  medium  it  traverses  is 
uniform  in  density. 

If  we  admit  a  sunbeam  through  a  small  opening  into  a  darkened  chamber, 
the  path  which  the  light  takes,  as  defined  by  means  of  the  dust  floating  in 
the  air,  is  a  straight  line. 

It  is  for  this"  reason  that  we  are  unable  to  see  through  a 
Wnat  practical 
applications  are    bent  tube,  as  we  can  through  a  straight  one. 

movement  ^at        ^n  taking  a™»  a^soi  w^h  a  gun  or  arrow,  we  proceed  upon 
light  in  straight    the  supposition  that  light  moves  in  straight  lines,  and  try  to 
make  the  projectile  go  to  the  desired  object  as  nearly  as  pos- 
sible by  the  path  along  which  the  light  comes  from  the  object  to  the  eye. 

FIG,  229. 


Thus,  in  Fig.  229,  the  line  A  B,  which  represents  the  line  of  sight,  is  also  the 
direction  of  a  line  of  light  passing  in  a  perfectly  straight  direction  from  the 
object  aimed  at  to  the  eyQ  of  the  marksman. 

A  carpenter  depends  upon  this  same  principle  for  the  purpose  of  determin- 
ing the  accuracy  of  his  work.  If  the  edge  of  the  plank  be  straight  and  uni- 
form, the  light  from  all  points  of  its  surface  will  come  to  the  eye  regularly  and 
uniformly ;  if  irregularities,  however,  exist,  they  will  cause  the  light  to  be 
irregular,  and  the  eye  at  once  notices  the  confusion  and  the  point  which  oc- 
casions it. 

what  is  a  ray        644.  A  ray  of  light  is  a  line  of  particles  of 
ofiight?        light,  or  the  straight   line  along  which  light 
passes  from  any  luminous  body. 

A  luminous  body  is  said  to  radiate  its  light,  because  the  light  issues  from 
it  in  every  direction  in  straight  lines. 


296 


WELLS'S   NATURAL   PHILOSOPHY. 


Whatisthela-w 
of  divergence? 


EX  lain     the  rays  of  light  radiate  from  any  lumin- 

divergence  of    ous  body,  they  diverge  from  one  another,  or 

rays  of  light.  .  ^ 7  * 

they  spread  over  more  space  as  they  recede 
from  their  source. 

Fig.  230  represents  the  manner  of  the  diverg-  FIG.  230. 

ence. 

The  surfaces  covered,  or 
illuminated  by  rays  of 
light  diverging  from  a  luminous  cen- 
ter, increase  as  the  squares  of  the 
distances. 

Thus,  a  candle  placed  behind  a  window  will  illuminate  a  certain  space  on 
the  wall  of  a  house  opposite.  If  the  wall  is  twice  as  far  from  the  candle  as 
from  the  window,  the  space  illuminated  by  it  will  be  four  times  as  large  as 
the  window.  If  the  wall  be  removed  to  three  times  the  distance,  the  surface 
covered  by  the  rays  of  light  will  be  nine  times  as  large,  and  so  on. 

A  collection  of  radiating  rays  of  light,  as  shown  in  Fig.  230,  constitutes 
what  is  called  a  "  pencil  of  light." 

A  thousand,  or  any  number  of  persons,  are  able  to  see  the 
same  object  at  the  same  time,  because  it  throws  off  from  its 
surface  an  infinite  number  of  rays  in  all  directions ;  and  one 
person  sees  one  portion  of  these  rays,  and  another  person 
another. 

Any  number  of  rays  of  light  are  able  to  cross  each  other,  in  the  same  space, 
without  jostling  or  interfering.  If  a  small  hole  be  made  from  one  room  to 
another  through  a  thin  screen,  any  number  of  candles  in  one  room  will  shine 
through  this  opening,  and  illuminate  as  many  spots  in  the  other  room  as  there 
are  candles  in  this,  all  their  rays  crossing  in  the  same  opening,  without  hinder- 
ance  or  diminution  of  intensity ;  just  as  sounds  of  different  character  proceed 
through  the  air  and  communicate  to  the  ear,  each  its  own  particular  tone, 
without  materially  interfering  with  each  other. 

Kays  of  light  which  continually  separate  as 
they  proceed  from  a  luminous  source,  are  called 
Diverging  Kays.  Kays  which  continually  ap- 
proach each  other  and  tend  to  unite  at  a  com- 
mon point,  are  called  Converging  Kays.  Kays  which  move 
in  parallel  lines,  are  called  Parallel  Rays. 

645.  When  rays  of  light,  radiated  from  a 
luminous  point,  through  the  surrounding 
space,  encounter  an  opaque  body,  they  will  (on  account 
of  their  transmission  in  straight  lines)  be  excluded  from 


Why  are  a 
great  number 
of  persons  able 
to  see  the  same 
object  at  the 
same  time? 


When  are  rays 
said  to  be  di- 
verging, con- 
verging, and 
parallel? 


What     is 

shadow? 


LIGHT. 


297 


Under  what 
circumstances 
will  the  size  of 
a  shadow  be 
increased  or 
diminished  f 


FIG.  231. 


the  space  behind  such  a  .body.     The  comparative  dark- 
ness thus  produced  is  called  a  shadow. 

When  the  light-giving  surface  is  greater  than  the  body  casting  the  shadow. 
a  cross  section  of  the  shadow  thrown  upon  a  plane  surface  will  be  less  than 
the  body ;  and  less,  moreover,  the  further  this  surface  is  from  the  body,  for 
the  shadowed  space  terminates  in  a  point. 

"When  the  luminous  center  is  smaller  than  the  opaque  body  casting  the 
shadow,  the  shadow  will  gradually  increase  in  size  with  the  distance,  without 
limit ;  thus  the  shadow  of  a  hand  held  near  a  candle,  and  between  a  candle 
and  the  wall,  is  gigantic. 

If  the  shadow  of  any  object  be  thrown  on  a  wall,  the  closer 
the  opaque  body  is  held  to  the  light-producing  center,  as  a 
candle,  for  example,  the  larger  will  be  its  shadow.     The  rea- 
son of  this  is,  that  the  rays  of  light  diverge  from  the  center 
in  straight  lines,  like  lines  drawn  from  the  center  of  a  circle ; 
and  therefore  the  nearer  the  object 
is  held  to  the  center,    the    greater 
the   number   of  rays    it  intercepts. 
Thus,  in  Fig.  231,  the  arrow  A,  held 
close  to  the  candle,  intercepts  a  large 
number  of  rays,  and   produces  the 
shadow  B  F;    while  the   same   ar- 
row held  at  C,  intercepts  a  smaller 
number  of  rays,  and  produces  only 
the  little  shadow  D  E. 

"When  two  or  more  luminous  ob- 
jects, not  in  the  same  straight  line, 
-J    shine  upon  the  same  object,  each  one 
•*   will  produce  a  shadow. 

646.   The  intensity  of  light  which  issues 
from  a  luminous  point  diminishes  in  the  same 
proportion  as  the  square  of  the  distance  from 
the  luminary  increases. 

Thus,  at  a  distance  of  two  feet,  the  intensity  of  light  will  be  one  fourth  of 
what  it  is  at  one  foot ;  at  three  feet  the  intensity  will  be  one  ninth  of  what  it 
is  at  one  foot.  In  other  words,  the  amount  of  illumination  at  the  distance  of 
one  foot  from  a  single  candle  would  be  the  same  as  that  from  four,  or  nine 
candles  at  a  distance  of  two  or  three  feet,  the  numbers  four  and  nine  being 
the  squares  of  the  distances  two,  and  three,  from  the  center  of  illumination. 

64?.  This  law,  therefore,  may  be  made  available  for  meas- 
uring the  relative  intensities  of  light  proceeding  from  different 
sources.  Thus,  in  order  to  ascertain  the  relative  quantities  of 
light  furnished  by  two  different  candles,  as,  for  example,  a 
wax  and  a  tallow  candle,  place  two  discs  or  sheets  of  white 
13* 


How  does  the 
intensity  of 
light  vary? 


Upon  what 
principle  may 
the  relative  in- 
tensities of 
different  lu- 
minous bodies 
be  ascertained  ? 


298  WELLS'S  NATURAL  PHILOSOPHY. 

paper,  a  few  feet  apart  on  a  wall,  and  throw  the  light  of  one  candle  on  one 
disc,  and  the  light  of  the  other  candle  upon  the  other  disc.  If  they  are  of 
unequal  illuminating  power,  the  candle  which  affords  the  most  light  must 
be  moved  back  until  the  two  discs  are  equally  illuminated.  Then,  by  meas- 
uring the  distance  between  each  candle  and  the  disc  it  illuminates,  the  lum- 
inous intensities  of  the  two  candles  may  be  calculated,  their  relative  intensi- 
ties being  as  the  squares  of  tneir  distances  from  the  illuminated  discs.  If, 
when  the  discs  are  equally  illuminated,  the  distance  from  one  candle  to  its 
disc  is  double  the  distance  of  the  other  candle  from  its  disc,  then  the  first 
candle  is  four  times  more  luminous  than  the  second ;  if  the  distance  be  triple, 
it  is  nine  times  more  luminous,  and  so  on. 

Instruments  called  "  Photometers,"  operating  in  a  similar  manner,  have  also 
been  constructed  for  measuring  the  relative  intensity  of  two  luminous  bodies. 
Their  arrangement  and  plau  of  operation  is  substantially  the  same  as  in  the 
method  described. 

648.  The  light  of  the  sun  greatly  exceeds  in 

What    is    the 

most    intense    intensity  that  derived  from  any  other  lumin- 

light  known?  ,       ;  * 

ous  body. 

The  most  brilliant  artificial  lights  yet  produced,  are  very  far  inferior  to  the 
splendor  of  the  solar  light,  and  when  placed  between  the  disc  of  the  sun  and 
the  eye  of  the  observer,  appear  as  black  spots. 

Dr.  "Wollaston  has  calculated  that  it  would  require  twenty  thousand  mil- 
lions of  the  brightest  stars  like  Sirius  to  equal  the  light  of  the  sun,  or  that 
that  orb  must  be  one  hundred  and  forty  thousand  times  further  from  us  than 
he  is  at  present,  to  be  reduced  to  the  illuminating  power  of  Sirius. 

The  light  of  the  full  moon  has  also  been  estimated  as  three  hundred  thou- 
sand times  less  intense  than  that  of  the  sun. 

During  the  day  the  intensity  of  the  sun's  light  is  so  great  as  to  entirely  eclipse 
that  of  the  stars,  and  render  them  invisible ;  and  for  the  same  reason,  we  only 
notice  the  light  emitted  by  fire-flies  and  phosphorescent  bodies  in  the  dark. 

Are  the  move-  649.  Light  does  not  pass  instantaneously 
SJSStaneousf  through  space,  but  requires  for  its  passage  from 

one  point  to  another  a  certain  interval  of  time, 
with  what  ve-  The  velocity  of  light  is  at  the  rate  of  about 
t?areV?oe8llght  one  hundred  and  ninety-two  thousand  miles  in 

a  second  of  time. 

Light  occupies  about  eight  minutes  in  traveling  from  the 
lustrations  of  sun  to  the  earth.  To  pass,  however,  from  the  planet 
1?  htT?elOCUy  °f  1-rranus  to  the  eartn>  **  would  require  an  interval  of  three 

hours. 

The  time  required  for  light  to  traverse  the  space  intervening  between  the 
nearest  fixed  star  and  the  earth,  has  been  estimated  at  SJyears ;  and  from 
the  farthest  nebula3,  a  period  of  several  hundred  years  would  be  requisite,  so 


LIGHT.  299 

immense  is  their  distance  from  our  earth.  If,  therefore,  one  of  the  remote  fixed 
stars  were  to-day  blotted  from  the  heavens,  several  generations  on  the  earth 
would  have  passed  away  before  the  obliteration  could  be  known  to  man. 

The  following  comparison  between  the  velocity  of  light  and  the  speed  of  a 
locomotive  engine  has  been  instituted : — Light  passes  from  the  sun  to  the 
earth  in  about  eight  minutes ;  a  locomotive  engine,  traveling  at  the  rate  of 
a  mile  in  a  minute,  would  require  npward  of  one  hundred,  and  eighty  years  to 
accomplish  the  same  journey. 

who  first  as-  650.  The  velocity  of  light  was  first  deter- 
mined  by  Von  Roemer,  an  eminent  Danish 
astronomer,  from  observations  on  the  satellites 

of  Jupiter. 

Ex  lain  the  The  metnod  ^y  ^icl1  V°n  Roemer  arrived  at  this  result 
method  by  may  be  explained  as  follows : — The  planet  Jupiter  is  sur- 
Tocitv  of  eii"ht  rounded  by  several  satellites,  or  moons,  which  revolve  about 
was  determined  it  in  certain  definite  times.  As  they  pass  behind  the  planet, 
of^piter^wS  they  disappear  from  the  sight  of  an  observer  on  the  earth,  or 
ellites.  in  other  words,  they  undergo  an  eclipse. 

The  earth  also  revolves  in  an  orbit  about  the  sun,  and  in  the  course  of  its 
revolution  is  brought  at  one  time  192  millions  of  miles  nearer  to  Jupiter  than 
it  is  at  another  time,  when  it  is  in  the  most  remote  part  of  its  orbit.  Suppose, 
now,  a  table  to  be  calculated  by  an  astronomer,  at  the  time  of  year  when  the 
earth  is  nearest  to  Jupiter,  showing,  for  twelve  successive  months,  the  exact 
moment  when  a  particular  satellite  would  be  observed  to  be  eclipsed  at  that 
point.  Six  months  afterward,  when  the  earth,  in  the  course  of  its  revolution, 
has  attained  a  point  192  millions  of  miles  more  remote  from  Jupiter  than  it 
formerly  occupied,  it  would  be  found  that  the  eclipse  of  the  satellite  would 
occur  sixteen  minutes,  or  9GO  seconds,  later  than  the  calculated  time.  This 
delay  is  occasioned  by  the  fact  that  the  light  has  had  to  pass  over  a  greater 
distance  before  reaching  the  earth  than  it  did  when  the  earth  was  in  the  op- 
posite part  of  its  orbit,  and  if  it  requires  sixteen  minutes  to  pass  over  192  mil- 
lions of  miles,  it  will  require  one  second  to  move  over  200,000  miles.  When, 
on  the  contrary,  the  earth  at  the  end  of  the  succeeding  six  months  has  as- 
sumed its  former  position,  and  is  192  millions  of  miles  nearer  Jupiter,  the 
eclipse  will  occur  sixteen  minutes  earlier,  or  at  the  exact  calculated  time  given 
in  the  tables.  The  velocity  of  light,  therefore,  in  round  numbers,  maybe  con- 
sidered as  200,000  miles  per  second.*  A  more  exact  calculation,  founded  on 
perfectly  accurate  data,  gives  as  the  true  velocity  of  light  192,500  miles  per 
second. 

*  The  explanation  above  given  will  be  made  clear  by  reference  to  the  following  dia- 
gram, Fig.  232.  S  represents  the  sun,  a  b  the  orbit  of  the  earth,  and  T  T'  the  position  of 
the  earth  at  different  and  opposite  points  of  its  orbit.  J  represents  Jupiter,  and  E  its 
satellite,  about  to  be  eclipsed  by  passing  within  the  shadow  of  the  planet.  Now  the  time 
of  the  commencement  or  termination  of  an  eclipse  of  the  satellite,  is  the  instant  at  which 
the  satellite  would  appear,  to  an  observer  on  the  earth,  to  enter,  or  emerge  from  the 


300  WELLS'S  NATURAL   PHILOSOPHY. 

Several  other  plans  have  been  devised  for  determining  the  velocity  of  light, 
the  results  of  which  agree  very  nearly  with  those  obtained  by  the  observations 
on  the  satellites  of  Jupiter.* 

when  IB  light  651.  When  a  ray  of  liglit  strikes  against  a 
surface,  and  is  caused  to  turn  back  or  rebound 

in  a  direction  different  from  whence  it  proceeded,  it  is  said 

to  be  reflected. 

what  is   ab-        652.  When  rays  of  light  are  retained  upon 

iTght?°Q  °f  tne  surface  upon  which  they  fall,  they  are  said 
to  be  absorbed  ;  in  consequence  of  which  their 

presence  is  not  made  sensible  by  reflection. 

The  question  as  to  what  becomes  of  the  light  which  is  absorbed  by  a  body, 
can  not  be  satisfactorily  answered.  In  all  probability  it  is  permanently  re- 
tained within  the  substance  of  the  absorbing  body,  since  a  body  which  absorbs 
light  by  continued  exposure,  does  not  radiate  or  distribute  it  again  hi  any 
way,  as  it  might  do  if  it  had  absorbed  heat. 

shadow  of  the  planet.  If  the  transmission  of  light  were  instantaneous,  it  is  obvious  that 
an  observer  at  T',  the  most  remote  part  of  the  earth's  orbit,  would  see  the  eclipse  begin 
and  end  at  the  same  moment  as  an  observer  at  T,  the  part  of  the  earth's  orbit  nearest  to 
Jupiter.  This,  however,  is  not  the  case,  but  the  observer  at  T'  sees  the  eclipse  960  sec- 
onds later  than  the  observer  at  T  ;  and  as  the  distance  between  these  two  stations  is  198 
millions  of  miles,  we  have,  as  the  velocity  of  light  in  one  second,  192,000,000-r-960= 
200,000. 

FIG.  232. 


*  A  yery  ingenious  plan  was  devised  a  few  years  since  by  M.  Fizeau  of  Paris,  by  which 
the  Telocity  of  artificial  light  was  determined  and  found  to  agree  with  that  of  solar  light. 
A  disc,  or  wheel,  carrying  a  certain  number  of  teeth  upon  its  circumference,  was  made  to 
revolve  at  a  known  rate  :  placing  a  tube  behind  these,  and  looking  at  the  open  spaces  be- 
tween the  teeth,  they  become  less  evident  to  sight,  the  greater  the  velocity  of  the  moving 
wheel,  until,  at  a  certain  speed,  the  whole  edge  appears  transparent.  The  ratu  at  which 
the  wheel  moves  being  known,  it  is  easy  to  determine  the  time  occupied  while  one  tooth 
passes  to  take  the  place  of  the  one  next  to  it.  A  ray  of  light  is  made  to  traverse  many 
miles  through  space,  and  then  passes  through  the  teeth  of  the  revolving  disc.  It  moves 
the  whole  distance  in  just  the  time  occupied  in  the  uiorement  of  a  single  tooth  to  the  plac« 
of  another  at  a  certain  speed. 


REFLECTION   OF   LIGHT.  301 


SECTION    I. 

REFLECTION     OP     LIGHT. 


what    occurs  «  When  rays  of  light  fall  upon  any  sur- 

upon  aS^su"!  ^ace?  tneJ  mav  be  reflected,  absorbed,  or 
face?  transmitted.  Only  a  portion  of  the  light, 

however,  which  meets  any  surface  is  reflected,  the  remain- 
der being  absorbed,  or  transmitted. 

when  does  a  654.  When  the  portion  of  light  reflected 
white  ana^S  ^rom  anv  surface,  or  point  of  a  surface,  to  the 
dark?  eye  js  considerable,  such  surface,  or  point,  ap- 

pears whito  ?  when  very  little  is  reflected,  it  appears  dark- 
colored;  but  when  all,  or  nearly  all  the  rays  are  absorbed,  and 
none  are  reflected  back  to  the  eye,  the  surface  appears  black. 

Thus,  charcoal  is  black,  because  it  absorbs  all  the  light  which  falls  upon  it, 
and  reflects  none.  Such  a  body  can  not  be  seen  unless  it  is  situated  near 
other  bodies  which  reflect  light  to  it. 

According  to  a  variation  in  the  manner  of  reflecting  light,  the  same  surface 
which  appears  white  to  an  eye  in  one  position,  may  appear  to  be  black  from 
another  point  of  view,  as  frequently  happens  in  the  case  of  a  mirror,  or  of 
any  other  bright,  or  reflecting  surface. 

what  are  good        Dense  bodies,   particularly  smooth  metals, 
i?c2t?°rs     °f    ren<ect  light  most  perfectly.     The  reflecting 
power  of  other  bodies  decreases  in  proportion 
to  their  porosity. 

HOW  are  mm-  655.  All  bodies  not  in  themselves  luminous, 
SSSSf  b£dsr  become  visible  by  reflecting  the  rays  of  light. 

l)le?  It  is  by  the  irregular  reflection  of  light  that  most  objects  in 

nature  are  rendered  visible  ;  since  it  is  by  rays  which  are  dispersed  from  re- 
flecting surfaces,  irregularly  and  in  every  direction,  that  bodies  not  exposed 
to  direct  light  are  illuminated.  If  light  were  only  reflected  regularly  from  the 
surface  of  non-luminous  bodies,  we  should  see  merely  the  image  of  the  lumin- 
ous object,  and  not  the  reflecting  surface.*  In  the  day-time,  the  image  of  the 
sun  would  be  reflected  from  the  surface  of  all  objects  around  us,  as  if  they 
were  composed  of  looking-glass,  but  the  objects  themselves  would  be  invisi- 
ble. A  room  in  which  artificial  lights  were  placed  would  reflect  these  lights 
from  the  walls  and  other  objects  as  if  they  were  mirrors,  and  all  that  would 
be  visible  would  be  the  multiplied  reflection  of  the  artificial  lights.  x  . 

*  In  a  very  good  mirror  we  scarcely  perceive  the  reflecting  surface  intervening  between 
us  and  the  images  it  shows  us. 


302 


WELLS'S  NATURAL   PHILOSOPHY. 


What  effect  has        Tlie  atmosPliere  reflects  light  irregularly,  and  every  particle 
the  atmosphere    of  air  is  a  luminous  center,  which  radiates  light  in  every  direc- 

tion'     Were  it;  not  for  this>  the  8un's  M^ht  would  only  illumi- 
nate those  spaces  which  are  directly  accessible  to  its  rays,  and 
darkness  would  instantly  succeed  the  disappearance  of  the  sun  below  the  horizon. 

656.  Any  surface  which  possesses  the  power 
of  reflecting  light  in  the  highest  degree  is  called 


What      is 
Mirror? 


a  MIRROR. 


Into  how  many 
classes  are  mir- 
rors divided  ? 


FIG.  233. 


What    is    the 


Mirrors  are  divided  into  three  general  classes, 
without  regard  to  the  material  of  which  they  con- 
sist, viz..  Plane,  Concave,  and  Convex  Mirrors. 

These  three  varieties  of  mirrors  are  represented  in  Fig. 
233;  A,  being  plane,  like  an  ordinary  looking-glass;  B, 
concave,  like  the  inside  of  a  watch-glass ;  and  C,  convex, 
like  the  outside  of  a  watch-glass. 

657.  When  light  falls  upon 
a  Plane  and   polished  surface, 
of  light?  faQ  angle  of  reflection  is  equal 

to  the  angle  of  incidence. 

This  is  the  great  general  law  which  governs  the  reflec- 
tion of  light,  and  is  the  same  as  that  which  governs  the 
motion  of  elastic  bodies. 

Thus,  in  Fig.  234,  let  A  B  be  the  direction  of  an  inci- 

FlG  234  dent  ray  of  light'  faUing  on  a  mirror>  F  °- 

'  It  will  be  reflected  in  the  direction  B  E. 

If  we  draw  a  line,  D  B,  perpendicular  to 
the  surface  of  the  mirror,  at  the  point  of 
reflection,  B,  it  will  be  found  that  the 
angle  of  incidence,  A  B  D,  is  precisely 
equal  to  the  angle  of  reflection,  E  B  D. 

The  same  law  holds  good  in 
regard  to  every  form  of  surface,  curved  as  well  as  plane, 

since  a  curve  may  be  supposed 
to  be  formed  of  an  infinite  num- 
ber of  little  planes. 

Thus,  in  Fig.  235,  the  incident  ray,  E  0, 
falling  upon  the  concave  surface,  a  C  &, 
will  still  be  reflected,  in  obedience  to  the 
same  law,  in  the  direction  C  D,  the  angle 
being  reckoned  from  the  perpendicular  to 
that  point  of  the  curve  where  the  incident 
ray  falls.  The  same  will  also  be  true  of 
the  convex  surface,  A  G  B. 


REFLECTION   OF    LIGHT/  303 

658.  An  image,  in  optics,  is  the  figure  of 
by  an  image  ?  any  object  made  by  rays  proceeding  from  the 
several  points  of  it. 

what  is  a  com  659.  A  common  looking-glass  consists  of  a 
3as9?lo°king~  glass  plate,  having  smooth  and  parallel  sur- 
faces, and  coated  on  the  back  with  an  amalgam*  of  tin 
and  quicksilver. 

The  images  formed  in  a  common  looking-glass 

Hovaretheim-  °  11,1  n       -•  J?Ut. 

ages  formed  in  are  mainly  produced  by  the  reflection  of  the 
rays  of  light  from  the  metallic  surface  attached 
to  the  back  of  the  glass,  and  not  from  the  glass  itself. 

The  effect  may  be  explained  as  follows: — A  portion  of  the  light  incident 
upon  the  anterior  surface  is  regularly  reflected,  and  another  portion  irregu- 
larly. The  first  produces  a  very  faint  image  of  an  object  placed  before  the 
glass,  while  the  other  renders  the  surface  of  the  glass  itself  visible.  Another, 
and  much  greater  portion,  however,  of  the  light  falling  upon  the  anterior  sur- 
face passes  into  the  glass  and  strikes  upon  the  brilliant  metallic  coating  upon 
the  back,  from  which  it  is  regularly  reflected,  and  returning  to  the  eye,  pro- 
duces a  strong  image  of  the  object  There  are,  therefore,  strictly  speaking, 
two  images  formed  in  every  looking-glass — the  first  a  faint  one  by  the  light 
reflected  regularly  from  the  anterior  surface,  and  the  second  a  strong  one  by 
the  light  reflected  from  the  metallic  surface ;  and  one  of  these  images  will  be 
before  the  other  at  a  distance  equal  to  the  thickness  of  the  glass.  In  good 
mirrors,  the  superior  brilliancy  of  the  image  produced  by  the  metallic  surface 
will  render  the  faint  image  produced  by  ttie  anterior  surface  invisible,  but  in 
glasses  badly  silvered,  the  two  images  may  be  easily  seen. 

If  the  surfaces  of  the  mirror  could  be  so  highly  polished  as  to  reflect  regu- 
larly all  the  light  incident  upon  it,  the  mirror  itself  would  be  invisible,  and  the 
observer,  receiving  the  reflected  light,  would  perceive  nothing  but  the  images 
of  the  objects  before  it.  This  amount  of  polish  it  is  impossible  to  effect  arti- 
ficially, but  in.  many  of  the  large  plate-glass  mirrors  manufactured  at  the  pres- 
ent time,  a  high  degree  of  perfection  is  attained.  Such  a  mirror  placed  ver- 
tically against  the  wall  of  a  room,  appears  to  the  eye  merely  as  an  opening 
leading  into  another  room,  precisely  similar  and  similarly  furnished  and  illum- 
inated ;  and  an  inattentive  observer  is  only  prevented  from  attempting  to 
walk  through  such  an  apparent  opening  by  encountering  his  own  image  as 
he  approaches  it. 

660.  A  plane  mirror  only  changes  the  direc- 

In  what  man-  L  *      .         ° 

ner    does    a    tion  of  the  r&ys  of  light  which  fall  upon  it, 

plane     mirror  i,       •  ,1      •  i     ,•  •.•  TJ? 

reflect  rays  of    without  altering  their  relative  position.      If 
they  fall  upon  it  perpendicularly,  they  will  be 

*  An  amalgam  is  a  mixture  or  compound  of  quicksilver  and  some  other  metal, 


304 


WELLS'S  NATURAL  PHILOSOPHY. 


When  will  the 
image  in  a 
looking-glass 
appear  distort- 
How  is  an  ap- 
parent change 
of  place  caused 
by  reflection  ? 


reflected  perpendicularly ;  if  they  fall  upon  it  obliquely, 
they  will  be  reflected  obliquely ;  the  angle  of  reflection 
being  always  equal  to  the  angle  of  incidence. 

If  the  two  surfaces  of  mirrors  are  not  parallel,  or  uneven, 
then  the  rays  of  light  falling  upon  it  wiU  not  be  reflected  regu- 
larly, and  the  image  will  appear  distorted. 

661.  We  always  seem  to  see  an  object  in  the 
direction  from  which  its  rays  enter  the  eye.    A 
mirror,  therefore,  which,  by  reflection,  changes 
the  direction  of  the  rays  proceeding  from  an 
object,  will  change  the  apparent  place  of  the  object. 

Thus,  if  the  rays  of  a  candle  fall  obliquely  upon  a  mirror,  and  are  reflected 
to  the  eye,  we  shall  seem  to  see  the  candle  in  the  mirror  in  the  direction 
in  which  they  proceed  after  reflection. 

If  we  lay  a  looking-glass  upon  the  floor,  with  its  face  uppermost,  and  place 
a  candle  beside  it,  the  image  of  the  candle  will  be  seen  in  the  mirror,  by  a 
person  standing  opposite,  as  inverted,  and  as  much  below  the  surface  of  the 
glass  as  the  candle  itself  stands  above  the  glass.  The  reason  of  this  is,  that 
the  incident  rays  from  the  candle  which  fall  upon  the  mirror  are  reflected  to 

the  eye  in  the  same 

FlG-  236'  direction    that    they 

would  have  taken,  had 
they  really  come  from 
a  candle  situated  as 
much  below  the  sur- 
face of  the  glass,  as 
the  first  candle  was 
above  the  surface. 
This  fact  will  be 
clearly  shown  by  re- 
ferring to  Fig.  236. 
When  we  look  into  a  plane  mirror  (the  common  looking-glass)  the  rays  of 
light  which  proceed  from  each  point  of  our  body  before  the  mirror  will,  after 
reflection,  proceed  as  if  they  came  from  a  point  holding  a  corresponding  posi- 
tion behind  the  mirror ;  and  therefore  produce  the  same  effect  upon  the  eye 
of  the  observer  as  if  they  had  actually  come  from  that  point.  The  image 
in  the  glass,  consequently,  appears  to  be  at  the  same  distance  behind  the 
surface  of  the  glass,  as  the  object  is  before  it. 

Let  A,  Fig.  237,  be  any  point  of  a  visible  object  placed  before  a  looking- 
glass,  M  N.  Let  A  B  and  A  C  be  two  rays  diverging  from  it,  and  reflected 
from  B  and  C  to  an  eye  at  0.  After  reflection  they  will  proceed  as  if  they 
had  issued  from  a  point,  a,  as  far  behind  the  surface  of  the  looking-glass 
as  A  is  before,  it — that  is  to  say,  the  distance  -A.  N  will  be  equal  to  th« 
distance  N  a. 


REFLECTION   OF   LIGHT. 


305 


For  this  reason  our  reflection  in 
a  mirror  seems  to  approach  us  when 
we  walk  toward  it,  and  to  retire 
from  us  as  we  retire. 

Upon  the  same  principle,  when 
trees,  buildings,  or  other  objects 
are  reflected  from  the  horizontal 
surface  of  a  pond,  or  other  smooth 
sheet  of  water,  they  appear 
verted,  since  the  light  of  the  obj 
reflected  to  our  eyes  from  the 
surface  of  the  water,  comes  to  us 
with  the  same  direction  as  it 
would  hare  done,  had  it  proceeded 
directly  from  an  inverted  object 
in  the  water. 

In  Fig.  238,  the  light  proceed- 


FIG.  237. 


FIG.  238. 


ing  from  the  arrow-head,  A,  strikes  the  water 
at  F,  and  is  reflected  to  D,  and  that  from 
the  barb,  B,  strikes  the  water  at  E,  and  is 
reflected  to  C.  A  spectator  standing  at  G 
will  see  the  reflected  rays,  E  G  and  F  G,  as 
if  they  proceeded  directly  from  C  and  D,  and 
the  image  of  the  arrow  will  appear  to  be  lo- 
cated at  C  D. 

It  is  in  accordance  with  the  law  that  tho 
angles  of  incidence  are  equal  to  the  angles  of 
reflection,  that  a  person  is  enabled  to  see  his  whole  figure  reflected  from  the 
surface  of  a  comparatively  small  mirror.     Thus,  in  Fig.  239,  let  a  person,  C  D, 
FIG.  239.  be  placed  at  a  suitable  distance  from  a  mir- 

ror, A  B.  The  rays  of  light,  C  A,  proceed- 
ing from  the  head  of  the  person,  fall  perpen- 
dicularly upon  tho  mirror,  and  are  therefore 
reflected  back  perpendicularly,  or  in  the 
same  line;  the  rays  B  D  proceeding  from 

D  ^     ~~  the  feet,  however,  fall  obliquely  upon  the 

mirror,  and  are  therefore  reflected  obliquely,  and  reach  the  eye  in  the  same 
direction  they  would  have  taken  had  they  proceeded  from  the  point  F  behind 
the  mirror. 

is  the  same  662.  The  quantity  of  light  reflected  from  a 
£fl"cteJ°2igiSi  given  surface,  is  not  the  same  at  all  angles,  or 
inclinations.  When  the  angle  or  inclination 
with  which  a  ray  of  light  strikes  upon  a  reflecting  surface 
is  great,  the  amount  of  light  reflected  to  the  eye  will  be 


306 


WELLS'S   NATURAL   PHILOSOPHY. 


considerable  ;  when  the  angle,  or  inclination  is  small,  the 
amount  of  light  reflected  will  be  diminished. 

Thus,  for  example,  when  light  falls  perpendicularly  upon  the  surface  of 
glass,  25  rays  out  of  1,000  are  returned ;  but  when  it  falls  at  an  angle  of 
85°,  550  rays  out  of  1,000  are  returned. 

Thus,  a  surface  of  unpolished  glass  produces  no  image  of  an  object  by  re- 
flection when  the  rays  fall  on  it  nearly  perpendicularly ;  but  if  the  flame  of  a 
candle  be  held  in  such  a  position  that  the  rays  fall  upon  the  surface  at  a  very 
small  angle,  a  distinct  image  of  it  will  be  seen. 

"We  have  in  this  an  explanation  of  th^fact,  that  a  spectator  standing  upon 
the  bank  of  a  river  sees  the  images  of  the  opposite  bank  and  the  objects  upon 
it  reflected  in  the  water  most  distinctly,  while  the  images  of  nearer  objects 
are  seen  imperfectly,  or  not  at  all.  Here  the  rays  coming  from  the  distant 
objects  strike  the  surface  of  the  water  very  obliquely,  and  a  sufficient  number 
are  reflected  to  make  a  sensible  impression  upon  the  eye ;  while  the  rays  pro- 
ceeding from  near  objects  strike  the  water  with  little  obliquity,  and  the  light 
reflected  is  not  sufficient  to  make  a  sensible  impression  upon  the  eye. 

This  fact  may  be  clearly  seen  by  reference  to  Fig.  240. 

FIG.  240. 


Let  S  be  the  position  of  the  spectator ;  0  and  B  the  position  of  distant 
objects.  The  rays  0  R  and  B  R  which  proceed  from  them,  strike  the  surface 
of  the  water  very  obliquely,  and  the  light  which  is  reflected  in  the  direction 
R  S  is  sufficient  to  make  a  sensible  impression  upon  the  eye.  But  in  regard 
to  objects,  such  as  A,  placed  near  the  spectator,  they  are  not  seen  reflected, 
because  the  rays  A  R'  which  proceed  from  them  strike  the  water  with  but 
little  obliquity ;  and  consequently,  the  part  of  their  light  which  is  reflected 
in  the  direction  R'  S,  toward  the  spectator,  is  not  sufficient  to  produce  a  sen- 
sible impression  upon  the  eye. 

what  is  the  663.  If  an  object  be  placed  between  two 
pSaiiei°f plane  plane  mirrors,  each  will  produce  a  reflected 
mirrors  ?  image,  and  will  also  repeat  the  one  reflected 
by  the  other — the  image  of  the  one  becoming  the  object 
for  the  other.  A  great  number  of  images  are  thus  pro- 


BEFLECTION   OF   LIGHT.  307 

duced,  and  if  the  light  were  not  gradually  weakened  by 
loss  at  each  successive  reflection,  the  number  would  be  in- 
finite. 

If  the  mirrors  are  placed  so  as  to  form  an  angle  with  each  other,  the  num- 
ber of  mutual  reflections  will  be  diminished,  proportionably  to  the  extent  of 
the  angle  formed  by  the  mirrors. 

.  The  construction  of  the  optical  instrument  called  the  Kalei- 

Kaleidoscope.  doscope  is  based  simply  upon  the  multiplication  of  an  image 
by  two  or  more  mirrors  inclined  toward  each  other.  It  con- 
sists of  a  tube  containing  two  or  more  narrow  strips  of  looking-glass,  which 
run  through  it  lengthwise,  and  are  generally  inclined  at  an  angle  of  about 
60°.  If  at  one  end  of  the  tube  a  number  of  small  pieces  of  colored  glass 
and  other  similar  objects  are  placed,  they  will  be  reflected  from  the  mirrors 
in  such  a  way  as  to  form  regular  and  most  elegant  combinations  of  figures. 
An  endless  variety  of  symmetrical  combinations  may  be  thus  formed,  since 
every  time  the  instrument  is  moved  or  shaken  the  objects  arrange  themselves 
differently,  and  a  new  figure  is  produced. 

Wh  does  the  Upon  tne  surface  of  smooth  water  the  sun,  when  it  is  nearly 
sun  appear  at  vertical,  as  at  noon,  appears  to  shine  upon  only  one  spot, 

at  °noniy  ^ne  a11  the  rest  of  the  water  aPPearing  dark-  The  reason  of  this 
point  upon  the  is,  that  the  rays  fall  at  various  degrees  of  obliquity  on  the 
water5?  water,  and  are  reflected  at  similar  angles ;  but  as  only  those 

which  meet  the  eye  of  the  spectator  are  visible,  the  whole  sur- 
face will  appear  dark,  except  at  the  point  where  the  reflection  occurs. 

241  Thus,  in  Fig.  241,  of  the  rays 

S  A,  S  B,  and  S  C,  only  the  ray 
S  C  meets  the  eye  of  the  specta- 
tor, D.  The  point  C,  therefore, 
will  appear  luminous  to  the  spec- 
tator D,  but  no  other  part  of  the 
surface. 

Another  curious  optical  pheno- 
menon is  seen  when  the  rays  of 
the  sun,  or  moon  fall  at  an  angle 
upon  the  surface  of  water  gently 
agitated  by  the  wind.  A  long, 
tremulous  path  of  light  seems  to 
be  formed  toward  the  eye  of  the 
spectator,  while  all  the  rest  of  the 
surface  appears  dark.  The  reason 
of  this  appearance  is,  that  every  little  wave,  in  an  extent  perhaps  of  miles,  has 
some  part  of  its  rounded  surface  with  the  direction  or  obliquity  which,  accord- 
ing to  the  required  relation  of  the  angles  of  incidence  and  reflection,  fits  it  to 
reflect  the  light  to  the  eye,  and  hence  every  wave  in  that  extent  sends  its  mo- 
mentary gleam,  which  is  succeeded  by  others. 


80S  WELLS'S   NATURAL   PHILOSOPHY. 

what  is  a  con-        664.  A  concave  mirror  may  be  considered 

cave  Mirror  ?  f, 

as  the  interior  surface  of  a  portion,  or  segment 
of  a  hollow  sphere. 

This  is  clearly  shown  in  Fig.  242. 

A  concave  mirror  may  be  represented  by  a  bright  spoon,  or  the  reflector  of 
a  lantern. 

iiowaraparai-  When  parallel  rays  of  light  fall  upon  the 
Ied1f?o5m1a2flcocn-  surface  of  a  concave  mirror,  they  are  reflected 
cave  mirror?  and  cause(j  to  converge  to  B,  point  half  way 
between  the  center  of  the  surface  and  the  center  of  the 
curve  of  the  mirror.  This  point  in  front  of  the  mirror  is 
called  the  principal  focus  of  the  mirror. 

Thus,  in  Fig.  242,  let  1,  2,  3,  4,  etc.,  be 
parallel  rays  falling  upon  a  concave  mir- 
ror; they  will,  after  reflection,  be  found  con 
verging  to  the  point  0,  the  principal  focus, 
•which  is  situated  half  way  between  the  a 
center  of  the  surface  of  the  mirror  and 
the  geometrical  center  of  the  curve  of  the 
mirror,  a. 

Why  are  con-  665.        ConCaVO    , 

SSaEfSS    mirrors  are  some-  7 

times     designated  \ 

as  "  Burning  Mirrors,"  since 
the  rays  of  the  sun  which  fall  upon  them  parallel,  are  re- 
flected and  converged  to  a  focus  (tire-place),  where  their 
light  and  heat  are  increased  in  as  great  a  degree  as  the 
area  of  the  mirror  exceeds  the  area  of  the  focus.* 

666.  Diverging  rays  of  light  issuing  from  a 

In  what  man-  ,          *i  i     '  PI 

ner  arediverg-  luminous  body  placed  at  the  center  of  the  curve 
fleeted  from  a  of  a  concave  spherical  mirror,  will  be  reflected 

concave     mir-  A  ./>  i  •    i      i          i  •  i 

ror  ?  back  to  the  same  point  from  which  they  diverged. 

*  A  burning  mirror,  20  inches  in  diameter,  constructed  of  plaster  of  Paris,  gilt  and  bur- 
nished, has  been  found  capable  of  igniting  tinder  at  a  distance  of  50  feet.  It  is  related 
that  Archimedes,  the  philosopher  of  Syracuse,  employed  burning  mirrors  200  years  before 
the  Christian  era,  to  destroy  the  besieging  navy  of  Marcellus,  the  Roman  consul ;  his 
mirror  was  probably  constructed  of  a  great  number  of  flat  pieces.  The  most  remarkable 
experiments,  however,  of  this  nature,  were  made  by  Buffon,  the  eminent  French  natural- 
ist, who  had  a  machine  composed  of  168  small  plane  mirrors,  so  arranged  that  they  all 
reflected  radiant  heat  to  the  same  focus.  By  means  of  this  combination  of  reflecting  sur- 
faces he  was  able  to  set  wood  on  lire  at  the  distance  of  209  feet,  to  melt  lead  at  100  feet, 
and  silver  at  50  feet. 


REFLECTION   OF   LIGHT. 


309 


FIG.  243.  Thus,  if  A  B,  Fig.  243,  were  a  concave  spheri- 

cal mirror,  of  which  C  were  the  center,  rays  issu- 
ing from  C  would,  in  obedience  to  the  law  that 
the  angle  of  incidence  and  reflection  are  equal, 
meet  again  at  C. 

Diverging  rays  falling  on  a  spheri- 
cal concave  mirror,  if  they  issue  from 
the  principal  focus,  half  way  between  the  center  of  the  sur- 
face and  the  center  of  the  curve  of  the  mirror,  will  be  re- 
flected in  parallel  lines. 

Thus,  in  Fig.  244,  if  F  represent  a  can-  -pIG  244 

die  placed  before  a  concave  mirror,  ABC, 

half  way  between  the  center  of  its  surface, 

B,  and  the  center  of  its  curve,  C,  its  rays, 

falling  upon  the  mirror,  will  be  reflected 

in  the  parallel  lines  d  efg  h. 

This  principle  is  taken  advantage  of  in 

the  arrangement  of  the  illuminating  and 

reflecting  apparatus  of  light-houses.     The  lamps  are  placed  before  a  concave 

mirror,  in  its  principal  focus,  and  the  rays  of  light  proceeding  from  them  are 

reflected  parallel  from  the  surface  of  the  mirror. 

"When  the  rays  issue  from  a  point,  P,  Fig. 
245,  beyond  the  center,  C,  of  the  curve  of  the 
mirror,  they  will,  after  reflection,  converge  to 
a  focus,  /  between  the  principal  focus,  F,  and 
the  center  of  the  curve,  C. 

On  the  contrary,  if  the  rays  issue  from  a 
point  between  the  principal  focus,  F,  and  the 
surface  of  the  mirror,  they  will  diverge  after 
reflection. 

667.  Images  are  formed  by  concave  mirrors 
'by  con!  in  the  same  manner  as  by  plane  ones,  but  they 
are  of  different  size  from  the  object,  their  gen- 
eral effect  being  to  produce  an  image  larger  than  the 
object. 

When  an  object  is  placed  between  a  concave 
mirror  and  its  principal  focus,  the  image  will 
appear  larger  than  the  object,  in  an  erect  posi- 
tion and  behind  the  mirror. 

This  will  be  apparent  from  Fig.  246.  Let  a  be  an  object  situated 
within  the  focus  of  the  mirror.  The  rays  from  its  extremities  will  fall 
divergent  on  the  mirror,  and  be  reflected  less  divergent  to  the  eye  at  fr, 


FIG.  245. 


How  are  images 
formed 
cave  mi 


When  will  the 
image  formed 
by  a  concave 
mirror  be  mag- 
nified ? 


310 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  246. 


as   though  they  proceeded  from  an  ob- 
ject behind  the  mirror,  as  at  h.      To  an 
eye    at    b    also,    the    image  will   appear 
.*'•**  ly       larger  than  the  object  a,  since  the  anglo 
i    of  vision  is  larger. 

If  the  rays  proceed  from  a  distant  body, 
at  E  D.,   Pig.   247,  beyond  the  cen- 
ter, C,  of  a  spherical  concave  mirror,  A  B, 
they  will,  after  reflection,  be  converged  to 
a  focus  in  front  of  the  mirror,  and  some- 
what nearer  to  the  center,  C,  than  the  prin- 
upon    any 


FIG.  247. 


cipal  focus,  and  there  paint 
substance  placed  to  receive  it,  an  im- 
age inverted,  and  smaller  than  the  object ; 
this  image  will  be  very  bright,  as  all 'the 
light  incident  upon  the  mirror  will  be  gath- 
ered into  a  small  space.  As  the  object 
approaches  the  mirror,  the  image  recedes 
from  it  and  approaches  C ;  and  when  situ- 
ated at  C,  the  center  of  the  curve  of  the  mirror,  the  image  will  be  reflected 
as  large  as  the  object ;  when  it  is  at  any  point  between  C  and  f,  supposing  / 
to  be  the  focus  for  parallel  rays,  it  will  be  reflected,  enlarged,  and  more  dis- 
tant from  the  mirror  than  the  object,  this  distance  increasing,  until  the  ob- 
ject arrives  at/  and  then  the  image  becomes  infinite,  the  rays  being  reflected 
parallel* 

668.  When  an  object  is  further  from  the 
surface  of  a  concave  mirror  than  its  principal 
focus,  the  image  will  appear  inverted  ;  hut 
when  the  object  is  between  the  mirror  and  its 
principal  focus,  the  image  will  be  upright,  and 

increase  in  size  in  proportion  as  the  object  is  placed  nearer 

to  the  focus. 

The  fact  that  images  are  formed  at  the  foci  of  a  concave  mirror,  and  that  by 
varying  the  distance  of  objects  before  the  surface  of  the  mirror,  we  may  vary 
the  position  and  size  of  the  images  formed  at  such  foci,  was  often  taken  ad- 
vantage of  in  the  middle  ages  to  astonish  and  delude  the  ignorant.  Thus, 
the  mirror  and  the  object  being  concealed  behind  a  curtain,  or  a  partition,  and 
the  object  strongly  illuminated,  the  rays  from  the  object  might  be  reflected 
from  the  mirror  in  such  a  manner  as  to  pass  through  an  opening  in  the  screen, 
and  come  to  a  focus  at  some  distance  beyond,  in  the  air.  If  a  cloud  of  smoke 

*  In  all  the  cases  referred  to,  of  the  reflection  of  light  from  concave  mirrors,  the  aper- 
ture or  curvature  of  the  mirror  is  presumed  to  be  inconsiderable.  If  it  be  increased  be- 
yond a  certain  limit,  the  rays  of  light  incident  upon  it  are  modified  in  their  reflection 
from  its  surface. 


When  will  the 
images  reflect- 
ed from  a  con- 
cave mirror 
appear  invert- 
ed, and  when 
erect? 


REFLECTION   OF   LIGHT. 


811 


from  burning  incense  were  caused  to  ascend  at  this  point,  an  image  would  be 
formed  upon  it,  and  appear  suspended  in  the  air  in  an  apparently  supernatural 
manner.  In  this  way,  terrifying  apparitions  of  skulls,  daggers,  etc.,  were 
produced. 

669.  A  Convex  Mirror  may  be  considered 
as  any  given  portion  of  the  exterior  surface  of 


What  is  a  Con- 
vex Mirror  ? 


a  sphere. 


Where  is  the 
principal  focus 
of  a  convex 
mirror? 


The  principal  focus  of  a  convex  mirror  lies 
as  far  behind  the  reflecting  surface  as  in  con- 
cave mirrors  it  lies  before  it.  (See  §  664) 
The  focus  in  this  case  is  called  the  virtual  focus,  because 
it  is  only  an  imaginary  point,  toward  which  the  rays  of 
reflection  appear  to  be  directed. 

Thus,  let  a  b  c  d  e,  Fig.  248,  be 
parallel  rays  incident  upon  a  convex 
mirror,  A  B,  whose  center  of  curvature 
is  C.  These  rays  are  reflected  diverg- 
ent, in  the  directions  a  &'  c'  d'  e,  as 
though  they  proceeded  from  a  point, 
F,  behind  the  mirror,  corresponding 
to  the  focus  of  a  concave  mirror. 

If  the  point  C  be  the  geometrical 
center  of  the  curve  of  the  mirror,  the 


How  are  di- 
verging and 
converging 
rays  reflected 
from  a  convex 
mirror  ? 

vergent. 

What  is  the 
nature  of  the 
images  formed 
by  convex  mir- 
rors? 


\^-^~ 

1 

f. 

I) 

tL 

t. 

point  F  will  be  half  way  between  C 
and  the  surface  of  the  mirror ;  as  this 
focus  is  only  apparent,  it  is  called  the  virtual  focus. 

Rays  of  light  falling  upon  a  convex  mirror, 
diverging,  are  rendered  still  more  divergent  by 
reflection  from  its  surface  ;  and  convergent 
rays  are  reflected,  either  parallel  or  less  con- 

FlG.  249. 

670.  The  general  effect 
of  convex  mirrors  is  to 
produce  an  image  smaller 
than  the  object  itself. 

Thus,  in  Fig.  249,  let  D  E  be  an  object  placed 
before  a  convex  mirror,  A  B ;  the  rays  proceed- 
ing from  it  will  be  reflected  from  the  convex  sur- 
face to  the  eye  at  H  K,  as  though  they  proceeded 
from  an  object,  d  e,  behind  the  mirror,  thus  pre- 
senting an  image  smaller,  erect,  and  much  nearer 
the  mirror  than  the  object. 


312  WELLS'S  NATURAL  PHILOSOPHY. 

Thus  the  globular  bottles  filled  with  colored  liquid,  in  the  window  of  a 
drug-store,  exhibit  all  the  variety  of  moving  scenery  without,  such  as  car- 
riages, carts,  and  people  moving  in  different  directions :  the  upper  half  of 
each  bottle  exhibiting  all  the  images  inverted,  while  the  lower  half  exhibits 
another  set  of  them  in  the  erect  position. 

Convex  mirrors  are  sometimes  called  dispersing  mirrors,  as  all  the  rays  of 
light  which  fall  upon  them  are  reflected  in  a  diverging  direction. 

what  is  ca-        671.    That  department  of  the   science  of 
toptncs?        optics  which  treats  of  reflected  light,  is  often 
designated  as  CATOPTRICS. 

SECTION   II. 

REFRACTION     OP     LIGHT. 

what  is  meant  Light  traverses  a,  given  transparent  sub- 
£y  th?. re£*c-  stance,  such  as  air.  water,  or  glass,  in  a  straight 

tion  of  light?  n         • 

line,  provided  no  reflection  occurs  and  there  is 
no  change  of  density  in  the  composition  of  the  medium  ; 
but  when  light  passes  obliquely  from  one  medium  to  an- 
other, or  from  one  part  of  the  same  medium  into  another 
part  of  a  different  density,  it  is  bent  from  a  straight  line, 
or  refracted. 

what  is  a  me-  672.  A  medium,  in  optics,  is  any  substance, 
dium  in  optics?  g^j^  \{^^  Or  gaseous,  through  which  light 
can  pass. 

A  medium,  in  optics,  is  said  to  be  dense  or  rare,  according  to  its  power  of 
refracting  light,  and  not  according  to  its  specific  gravity.  Thus  alcohol,  olive 
oil,  oil  of  turpentine,  and  the  like  substances,  although  of  less  specific  gravity 
than  water,  have  a  greater  refractive  power ;  they  are,  therefore,  called  denser 
media  than  water. 

673.  The  fundamental  laws  which  govern  the  refraction  of  light  may  be 
stated  as  follows : 

what  laws  gor-  When  light  passes  from  one  medium  into 
Sra Sought?0"  an°ther,  in  a  direction  perpendicular  to  the 
surface,  it  continues  on  in  a  straight  line,  with- 
out altering  its  course.  When  light  passes  obliquely  from 
a  rarer  into  a  denser  medium,  it  is  refracted  toward  a 
perpendicular  to  the  surface,  and  this  refraction  is  in- 
creased or  diminished  in  proportion  as  the  rays  fall  more 
or  less  obliquely  upon  the  refracting  surface. 


REFRACTION   OF   LIGHT. 


313 


250, 


When  light  passes  obliquely  out  of  a  denser  into  a  rarer 
medium,  it  passes  through  the  rarer  medium  in  a  more 
oblique  direction,  and  further  from,  a  perpendicular  to  the 
surface  of  the  denser  medium. 

Thus,  in  Fig.  250,  suppose  n  m  to  represent  the 
surface  of  water,  and  S  0  a  ray  of  light  striking 
upon  its  surface.  When  the  ray  S  0  enters  tho 
water,  it  will  no  longer  pursue  a  straight  course, 
but  will  be  refracted,  or  bent  toward  the  perpen- 
dicular line,  A  B,  in  the  direction  S  0.  The  denser 
the  water  or  other  fluid  may  be,  the  more  the  ray 
S  O  H  will  be  refracted,  or  turned  toward  A  B. 
If,  on  the  contrary,  a  ray  of  light,  II  0,  passes  from 
the  water  into  the  air,  its  direction  after  leaving  tho  water  will  be  further  from 
the  perpendicular  A  0,  in  the  direction  0  S. 

The  effects  of  the  refraction  of  FIG.  251. 

light  may  be  illustrated  by  the  fol- 
lowing simple  experimenc  :  —  Let  a 
coin  or  any  other  object  be  placed 
at  the  bottom  of  a  bowl,  as  at  m, 
Fig.  251,  in  such  a  manner  that  the 
feyo  at  a  can  not  perceive  it,  on  ac- 
count of  the  edge  of  the  bowl  which 
intervenes  and  obstructs  the  rays  of 
light.  If  now  an  attendant  care- 
fully pours  water  into  the  vessel,  the 

coin  rises  into  view,  just  as  if  the  bottom  of  the  basin  had  been  elevated 
above  its  real  level.  This  is  owing  to  a  refraction  by  the  water  of  the  rays 
of  light  proceeding  from  the  coin,  which  are  thereby  caused  to  pass  to  tho 
eye  in  the  direction  1  1.  The  image  of  the  coin,  therefore,  appears  at  n,  in  the 
direction  of  these  rays,  instead  of  at  m.  its  true  position. 

A  straight  stick,  partly  immersed  in  water,  appears  to  be  broken  or  bent 
at  the  point  of  immersion.     This  is  owing  to  the  fact  that  the  rays  of  light 
proceeding  from  the  part  of  the  stick  contained  in  the  water  are  refracted,  or 
FIG.  252.         caused  to  deviate  from  a  straight  line  as  they  pass  from  the 
water  into  the  air  ;  consequently  that  portion  of  the  stick 
immersed  in  the  water  will  appear  to  be  lifted  up,  or  to 
be  bent  in  such  a  manner  as  to  form  an  angle  with  tho 
part  out  of  the  water. 

The  bent  appearance  of  the  stick  in  water  is  represented 
in  Fig.  252.  For  the  same  reason,  a  spoon  in  a  glass  of 
water,  or  an  oar  partially  immersed  in  water,  always  ap- 
pears bent. 

On  account  of  this  bending  of  light  from  objects  under  water,  a  person  who 
endeavors  to  strike  a  fish  with  a  spear,  must,  unless  directly  above  the  fisb, 

14 


314  WELLS'S  NATURAL  PHILOSOPHY. 

aim  at  a  point  apparently  below  it,  otherwise  the  weapon  will  miss,  by  pass- 
ing too  high. 

A  river,  or  any  clear  water  viewed  obliquely  from  the  bank,  appears  more 
shallow  than  it  really  is,  since  the  light  proceeding  from  the  objects  at  tho 
bottom,  is  refracted  as  it  emerges  from  the  surface  of  the  water.  The  depth 
of  water,  under  such  circumstances,  is  about  one  third  more  than  it  appears, 
and  owing  to  this  optical  deception,  persons  in  bathing  are  'liable  to  get  be- 
yond their  depth. 

Light,  on  entering  the  atmosphere,  is  re- 

Whatisatmos-  °       \  i  i  • 

pheric  refrac-  iractecl  in  a  greater  or  less  degree,  in  propor- 
tion to  the  density  of  the  air ;  consequently, 
as  that  portion  of  the  atmosphere  nearest  the  surface 
of  the  earth  possesses  the  greatest  density,  it  must  also 
possess  the  greatest  refractive  power. 

What  effect  has  From  tlli3  cause  the  sun  and  other  celestial  bodies  are  never 
refraction  upon  seen  in  their  true  situations,  unless  they  happen  to  be  verti- 
tht  sun  ¥ 10U  °f  ^ '  and  *ke  nearer  tnev  are  to  the  horizon,  the  greater  will 

be  the  influence  of  refraction  in  altering  the  apparent  place  of 
any  of  these  luminaries. 

This  forms  one  of  the  sources  of  error  to  be  allowed  for  in  all  astronomical 
observations,  and  tables  are  calculated  for  finding  the  amount  of  refraction, 
depending  on  the  apparent  altitude  of  tho  object,  and  the  state  of  the  barom- 
eter and  thermometer.  "When  the  object  is  vertical,  or  nearly  so,  this  error 
is  hardly  sensible,  but  increases  rapidly  as  it  approaches  the  horizon ;  so  that, 
in  the  morning,  the  sun  ia  rendered  visible  before  he  has  actually  risen,  and 
in  the  evening,  after  he  has  set. 

For  the  same  reason,  morning  does  not  occur  at  the  in-, 
cause  of  twi-  stant  of  ttie  aun'a  appearance  above  the  horizon,  or  night 
light?  set  in  as  soon  as  he  has  disappeared  below  it.  But  both 

at  morning  and  evening,  the  rays  proceeding  from  the  sun 
below  the  horizon  are,  in  consequence  of  atmospheric  refraction,  bent 
down  to  the  surface  of  the  earth,  and  thus,  in  connection  with  a  reflect- 
ing action  of  the  particles  of  the  ah*,  produce  a  lengthening  of  the  day,  termed 
twilight. 

In  what  man.  ^s  the  densitv  °^  ^e  air  diminishes  gradually  upward  from 
ner  is  light  re-  the  earth,  atmospheric  refraction  is  not  a  sudden  change  of 

direction>  as  in  the  case  of  the  passage  of  light  from  air  into 

water,  but  the  ray  of  light  actually  describes  a  curve,  being 
refracted  more  and  more  at  each  step  of  its  progress.  This  applies  to 
the  light  received  from  a  distant  object  on  the  surface  of  the  earth,  which  ia 
Ipwer  or  higher  than  the  eye,  as  well  as  to  that  received  from  a  celestial  ob- 
ject, since  it  must  pass  through  air  constantly  increasing  or  diminishing  in 
Density.  Hence,  in  the  engineering  operation  of  leveling,  this  refraction  must 
be  taken  into  consideration. 


REFRACTION  OF  LIGHT. 


315. 


,  674.  The  application  of  the  laws  of  refraction  of  light  ac- 

phenomena  of  count  for  many  curious  deceptive  appearances  in  the  at- 
Mirage.  mosphere,  which  are  included  under  the  general  name  of 

Mirage.  In  these  phenomena,  the  images  of  objects  far  remote  are  seen  at  an 
elevation  in  the  atmosphere,  either  erect  or  inverted.  Thus  travelers  upon  a 
desert,  where  the  surface  of  the  earth  is  highly  heated  by  the  sun,  are  often 
deceived  by  the  appearance  of  water  in  the  distance,  surrounded  by  trees  and 
villages.  In  the  same  manner  at  sea,  the  images  of  vessels  at  a  great  distance 
and  below  the  horizon,  will  at  times  appear  floating  in  the  atmosphere.  Such 
appearances  are  frequently  seen  with  great  distinctness  upon  the  great  Amer- 
ican lakes.  These  phenomena  appear  to  be  due  to  a  change  in  the  density  of 
the  strata  of  air  which  are  immediately  in  contact  with  the  surface  of  the  earth. 
Thus  it  often  happens  that  strata  resting  upon  the  land  may  be  rendered  much 
hotter,  and  those  resting  upon  the  water  much  cooler,  by  contact  with  the 
surface,  than  other  strata  occupying  more  elevated  positions.  Rays,  there- 
fore, on  proceeding  from  a  distant  object  and  traversing  these  strata,  will  be 
unequally  reflected,  and  caused  to  proceed  in  a  curvilinear  direction ;  and  in 
this  way  an  object  situated  behind  a  hill,  or  below  the  horizon,  may  be 

brought  into  view  and  appear  suspend- 
•^  ed  in  the  air.  This  may  be  readily 
understood  by  reference  to  Fig.  253. 
Suppose  the  rays  of  light  from  the 
ship,  S,  below  the  horizon  to  reach 
the  eye,  after  assuming  a  curvilinear 
direction  by  passing  through  strata  of 
air  of  varying  density;  then,  as  an 
object  always  appears  in  the  direction 
in  which  the  last  rays  proceeding  from 
it  enter  the  eye,  two  images  will  be  seen  in  the  direction  of  the  dotted 
lines,  one  of  them  being  inverted. 

These  phenomena  may  be  sometimes  imitated.  Thus,  if  we  look  along  a 
red  hot  bar  of  iron,  or  a  mass  of  heated  charcoal  at  some  image,  a  short  dis- 
tance from  it,  an  inverted  reflection  of  it  will  be  seen.  In  the  same  manner, 
if  we  place  in  a  glass  vessel  liquids  of  different  densities,  so  that  they  float 
one  above  another,  and  look  through  them  at  some  object,  it  will  be  seen 
distorted  and  removed  from  its  true  place,  by  reason  of  the  unequal  refractive 
and  reflective  powers  of  the  liquids  employed. 

675.  The  angle  of  refraction  of  light  is  not, 
like  the  angle  of  reflection,  equal  to  the  angle 
of  incidence ;  but  it  is  nevertheless  subject 
to  a  definite  law,  which  is  called  the  law  of 


Is  the  angle 
of  refraction 
equal  to  the 
anple  of  inci- 
dence? 


sines. 

What  is  a  sine? 


A  sine  is  a  right  line  drawn  from  any  point  in  one  of  the 
lines  inclosing  an  angle,  perpendicular  to  tho  other  line. 


316 


WELLS'S   NATURAL   PHILOSOPHY. 


FiG.  255. 


FIG.  254.  Thus,  in  Fig.  244,  let  A  B.  C  be  an  angle ;  then 

a  will  be  the  sine  of  that  angle,  being  drawn  from 
a  point  in  the  line  A  B,  perpendicular  to  the  line 
B  C.  Two  angles  may  be  compared  by  means  of 
their  sines,  but  whenever  this  is  done,  the  lengths 
of  the  sides  of  the  angles  must  be  made  equal,  be- 
cause the  sine  varies  in  length  according  to  the  length  of  the  lines  forming 
the  angle. 
-  The  general  law  of  refraction  is  as  follows: — 

What  is  the         When  a  ray  of  light  passes  from  one  medium 
general  law  of    to  another,  the  sine  of  the  angle  of  incidence 

refraction?  .      .  ° 

is  in  a  constant  ratio  to  the  sine  of  the  angle 
of  refraction. 

The  proportion  or  relation  between  these  sines  differs  when  different  media 
are  used ;   but  for  the  same  medium  it  is  always  the  same. 

Thus,  in  Fig.  255,  let  F  E  be  the  surface  of 
some  refracting  medium,  as  water,  and  H  R,. 
H'  R,  rays  incident  upon  it,  at  different  angles ; 
i  the  former  will  be  refracted  in  the  direction 
R  I' ;  a  and  6  will  be  the  sines  of  the  angle 
of  incidence,  and  c  d  the  sines  of  the  angle  of 
refraction ;  and  the  quotient  arising  from  di- 
viding 6  by  c,  is  the  same  as  that  from  divid- 
ing a  by  d.  In  the  case  of  air  and  water, 
the  sine  of  the  angle  of  incidence  in  the  air 
will  be  to  the  sine  of  the  angle  of  refraction 
in  water  as  4  is  to  3 ;  in  any  two  other  me- 
dia, a  different  ratio  would  be  observed  with  equal  constancy. 

The  quotient  found  by  dividing  the  sine  of 

What   is    the  ^  .       .  J  .      ° 

index  of  re-    the  angle  01  incidence  by  the  sine  of  the  angle 
of  refraction,  is  called  the  index  of  refraction. 

As  different  bodies  have  different  refractive  powers,  they  will  present  dif- 
ferent indices,  but  in  the  same  substance  it  is  always  constant.  Thus,  tho 
refractive  index  of  water  is  1.335,  of  flint  glass,  1.55,  of  the  diamond,  2.487. 
Is  light  ever  ^°  surface  ever  transmits  all  the  light  which  falls  upon  it, 
wholly  trans-  but  a  portion  is  always  reflected.  If,  in  a  dark  room,  wo 
allow  a  sunbeam  to  fall  on  the  surface  of  water,  the  division 
of  the  light  into  a  reflected  and  refracted  ray  will  be  clearly  perceptible. 

When  the  obliquity  of  an  incident  ray  passing  through  a 
cum  stances  will  denser  medium  toward  a  rarer  (as  through  water  into  air),  is 
such  that  the  sine  of  its  refracting  angle  is  equal  to  90°,  it 
ceases  to  pass  out,  and  is  reflected  from  the  surface  of  the 
denser  medium  back  into  it  again.  This  constitutes  the  only  known  instance 
of  the  total  reflection  of  light.  The  phenomenon  may  be  seen  by  looking 


EEFR ACTION  OF  LIGHT.  317 

through  the  sides  of  a  tumbler  containing  water,  up  to  the  surface  in  an 
oblique  direction,  when  the  surface  will  be  seen  to  be  opaque,  and  more  re- 
flective than  any  mirror,  appearing  like  a  sheet  of  burnished  silver. 

No  law  has  yet  been  discovered  which  will  -enable  us  to 
JudSe  of  the  refractive  power  of  bodies  from  their  other  quali- 
ence  the  re-  ties.  As  a  general  rule,  dense  bodies  have  a  greater  refrac- 
ofab6diesP?OWer  tive  power  than  those  which  are  rare;  and  the  refractive 
power  of  any  particular  substance  is  increased  or  diminished 
in  the  same  ratio  as  its  density  is  increased  or  diminished.  Eefractive  power 
seems  to  be  the  only  property,  except  weight,  which  is  unaltered  by  chemical 
combination ;  so  that  by  knowing  the  refractive  power  of  the  ingredients,  we 
can  calculate  that  of  the  compound. 

All  highly  inflammable  bodies,  such  as  oils,  hydrogen,  the  diamond,  phos- 
phorus, sulphur,  amber,  camphor,  etc.,  have  a  refractive  power  from  ten  to 
seven  times  greater  than  that  of  incombustible  substances  of  equal  density. 

Of  all  transparent  bodies  the  diamond  possesses  the 
greatest  refractive  or  light-bending  power,  although  it  is 
exceeded  by  a  few  deeply-colored,  almost  opaque  miner- 
als. It  is  in  great  part  from  this  property  that  the  dia- 
mond owes  its  brilliancy  as  a  jewel. 

Many  years  before  the  combustibility  of  the  diamond  was  proved  by  ex- 
periment, Sir  Isaac  Newton  predicted,  from  the  circumstance  of  its  high  re- 
fractive power,  that  it  would  ultimately  be  found  to  be  inflammable. 

If  the  surface  of  any  naturally. transparent  body  is  made 
rough  and  irregular,  the  rays  of  light  which  fall  upon  it 
are  refracted  and  reflected  so  irregularly,  that  they  fail  to 
penetrate  and  pass  through  the  substance  of  the  body, 
and  its  transparency  is  thus  destroyed. 

Glass  made  rough  on  its  surface  loses  its  transparency;  but  if  we  rub  a 
ground  glass  surface  with  wax,  or  any  other  substance  of  nearly  the  same 
optical  density,  we  fill  up  the  irregularities  and  restore  its  transparency.  Horn 
is  translucent,  but  a  horn  shaving  is  nearly  opaque.  The  reason  of  this  is 
that  the  surface  of  the  shaving  has  been  torn  and  rendered  rough,  and  the 
rays  of  light  falling  upon  it  are  too  much  reflected  and  refracted  to  be  trans- 
mitted, and  thereby  render  it  translucent.  On  the  same  principle,  by  filling 
up  the  pores  and  irregularities  of  the  surface  of  white  paper,  which  is  opaque, 
with  oil,  we  render  it  nearly  transparent. 

HOW  is  refrac-  According  to  the  undulatory  theory  of  light, 
won  accounted  refraction  is  supposed  to  be  due  to  an  altera- 
tion in  the  velocity  with  which  the  ray  of  light 
travels.  According  to  the  corpuscular  theory,  it  is  ac- 
counted for  on  the  supposition  that  different  substances 


318  WELLS'S   NATURAL   PHILOSOPHY. 

exert  different  attractive  influences  on  the  particles  of 

light  coming  in  contact  with  them. 

whatisDiop-        That  department   of  the  science  of  optics 

which  treats  of  the  refraction  of  light  is  termed 
Dioptrics. 

what  ensues  676.  When  a  ray  of  light  passes  through  a 
^MSM  through  transparent  medium  whose  sides  where  the 
Sif/BuTface^T  rav  enters  and  emerges  are  parallel,  it  will 

suffer  no  permanent  change  of  direction  by 
refraction,  since  the  second  surface  exactly  compensates 
for  the  refractive  effect  of  the  first. 


FIG,  256,  Tnus  let  A  •A-»  ^fe-  256»  be  a  Plato  of 

v  g  glass,  whose  sides  are  parallel,  and  B  C  a 

\^  ray  of  light  incident  upon  it  ;  it  will  be  re- 

>v  tg^m  fracted  in  the  direction  C  D,  and  on  leaving 

>w  the  glass  will  be  refracted  again,  emerging 

__•'  >!.•  *n  ^e  ^e  ^  ^'  Para^el  t°  *ne  course  it 

would  have  pursued  if  it  had  not  been  re- 
HD  fracted  at  all,  and  which  is  shown  by  the 

B\  dotted  line.     A  small  lateral  displacement  is, 

I  ^^  however,  occasioned  in  the  path  of  the  ray, 

*%^  depending  on  the  thickness    of  the   glass 

plate. 

This  explains  the  reason  why  a  plate  of 
glass  in  a  window  whose  surfaces  are  perfectly  parallel,  occasions  no  distor- 
tion, or  alteration  of  the  position  of  objects  seen  through  it,  by  reason  of  its 
refractive  power.  The  rays  suffer  two  refractions  in  contrary  directions,  which 
produce  the  same  effect  as  if  no  refraction  had  taken  place. 

what  happens  1^  the  surfaces  of  the  medium  through  which 
pa^L  thrSfgh  ^g^  passes  are  not  parallel,  the  direction  of 
£a?esaTenoet  evei7  rav  passing  through  it  is  permanently 
parallel?  altered,  the  change  being  greater  as  the  incli- 

nation of  the  two  surfaces  is  greater. 

Thus  window-glass  of  unequal  thickness  displaces  and  distorts  all  objects 
seen  through  it.  Hence  the  singular  distortion  of  objects  viewed  through  that 
swelling,  or  lump  of  glass  known  as  the  "bull's  eye,"  which  is  sometimes 
seen  in  the  center  of  very  coarse  panes  of  glass,  and  which  remains  where 
flie  glass-blower's  instrument  was  attached. 

is   a         677.  ADV  g^ass  having  two  plane  surfaces 
prism?         not  paraiie^  js  called  a  PRISM. 

'  —  ,  ••      <*r-"' 


REFRACTION   OF   LIGHT.  319 

•    As  ordinarily  constructed,  a  prism  is  an  FIG.  257. 

oblong,  triangular,  or  wedge-shaped  piece  of 
glass,  with  sides  inclined  at  any  angle,  as 
is  represented  in  Fig.  257. 

Explain  the  ac-  °n    l°°^^     timjUSh     & 

tion  of  the  prism,  all  objects  are  seen 
prism.  removed  from  their  true 

place.     Thus,  let  CAB,  Fig.  258,  be  a  prism,  and  D  E  a  ray  of  light  inci- 
FIG.  258.  dent  upon  it ;  it  will  be  refracted  in 

the  direction  E  F,  and  on  emerging, 

c  o-..  /\  witt  again  be  refracted  in  the  direc- 

tion F  H;  and  as  objects  always 

E/    ^p  appear  in  the  direction  in  which  the 

last  ray  enters  the  eye,  the  object 
D  will  appear  at  G,  in  the  direction 
of  the  dotted  line,  elevated  above  its 
real  position.  If  the  refracting  angle,  A  C  B,  had  been  placed  downward, 
the  object  would  have  appeared  as  much  depressed. 

The  prism,  although  of  simple  construction,  is  one  of  the  most  important 
of  optical  instruments,  and  to  its  agency  we  are  indebted  for  most  of  the  in- 
formation we  possess  respecting  the  nature  and  constitution  of  light.  Tho 
beautiful  and  complicated  results  of  its  practical  application  belong  to  that 
department  of  optics  which  treats  of  the  phenomena  of  color. 

678.  A  LENS  is  a  piece  6f  glass  or  other 
transparent  substance,  bounded  on  both  sides 
by  polished  spherical  surfaces,  or  on  the  one  side  by  a 
spherical,  and  on  the  other  by  a  plane  surface.  Kays  of 
light  passing  through  it  are  made  to  change  their  direc- 
tion, and  to  magnify  or  diminish  the  appearance  of  objects 
at  a  certain  distance. 

HOW  many  There  are  six  different  kinds  of  simple  lenses, 
Sn^sar? there?  a11  of  which  may  be  considered  as  portions  of 
the  external  or  internal  surface  of  a  sphere. 
Four  of  these  lenses  are  bounded  by  two  spherical  sur- 
faces, and  two  by  a  plain  and  spherical  surface. 

,    Fig.  259  represents  sectional  views  of  the  six  varieties  of  simple  lenses. 

•Explain the dif-       A  double  convex  lens  is  bounded  by  two 
ienses.kindsof    convex  spherical  surfaces,  as  at  A,  Fig,  259. 

To  this  figure  the  appellation  of  lens  was  first,  applied  from 
its  resemblance  to  a  lentil  seed  (in  Latin,  lens). 

A  plano-convex,  ;.or   single    convex  lens  has  one  side 


320 


WELLS'S  NATURAL  PHILOSOPHY. 


bounded  by  a  plane  surface,  and  the  other  by  a  convex 
surface.    It  is  represented  at  B,  Fig.  259. 


A  meniscus,  or  concavo-convex  lens  is  convex  on  one 
side  and  concave  on  the  other,  as  at  C,  Fig.  259. 

To  this  kind  of  lens  the  term  "  periscopic"  has  recently  been  applied,  from 
the  Greek,  signifying  to  view  on  all  sides. 

A  double  concave  lens  is  concave  upon  both  sides,  as 
at  D,  Fig.  259. 

A  plano-concave,  or  single  concave  lens,  is  bounded  on 
one  side  by  a  plane,  and  on  the  other  by  a  concave  sur- 
face, as  at  E,  Fig.  259. 

A  concavo-convex  lens  is  bounded  on  one  side  by  a 
concave,  and  on  the  other  by  a  convex  surface,  as  at  F, 
Fig.  259. 

into  ho*  many  The  six  varieties  of  simple  lenses  are  divided 
bem<E  into  two  classes,  which  are  denominated  con- 
verging  and  diverging  lenses,  since  the  one 
class  renders  parallel  rays  of  light  falling  upon  them  con- 
vergent, and  the  other  class  renders  them  divergent. 

In  Fig.  259  A  B  C  are  converging,  or  collecting  lenses,  and  D  E  F  diverg- 
ing, or  dispersing  lenses.  The  former  are  thickest  at  the  center  ;  the  latter 
are  thinner  at  the  center  than  at  the  edges. 

In  the  first  class  it  is  sufficient  to  consider  only  the  double-convex  lens, 
and  in  the  secqnd  class  only  the  double-concave  lens,  since  the  properties  of 
each  of  these  lenses  apply  to  all  the  others  of  the  same  class. 

For  optical  purposes  lenses  are  generally  made  of  glass,  but  in  some 
instances  other  substances  are  employed,  such  as  rock-crystal,  the  dia- 
mond, etc. 

^n  a^  the  various  kinds  of  lenses  there  must 
ke  a  Pomt  through  which  rays  of  light  passing 
experience  no  deviation  ;  or  in  other  words, 


m»t  is  the 


REFRACTION   OF   LIGHT.  321 

the  incident  and  emergent  rays  are  parallel.     Such  a  point 
is  called  the  optical  center  of  a  lens. 
what  is  the        The  axis  of  a  lens  is  a  straight  line  passing 
axis  of  a  lens?    through  the  center  perpendicular  to  the  sur- 
face of  the  lens. 

On  this  line  will  be  situated  the  geometrical  centers  of  tho 
considered  ex-     two  surfaces  of  the  lens,  or  rather  of  the  spheres  of  which 


actly  oentered  ?     they  form  portioT1g. 

A  lens  is  said  to  be  truly  or  .exactly  centered  when  its  optical  center  is  sit- 
uated at  a  point  on  the  axis  equally  distant  from  corresponding  .parts  of  the 
surface  in  every  direction  ;  as  then  objects  seen  through  the  lens  will  not  ap- 
pear altered  in  position  when  it  is  turned  round  perpendiciilarly  to  its  axis. 

in  what  man-        6^9-  Parallel  rays  of  light  falling  upon  a 
?aySare  affected    double-convex  lens  are  converged  to  a  focus 
ieyns?  °onvex    at  a  distance  varying  with  the  curvature  of 
its  sides. 

FIG.  260.  The  double-convex  lens  may  be  regarded  as 

two  prisms,  with  curved  surfaces,  united  at 
their  bases,  as  is  represented  in  Fig.  260  ; 
and  as  in  a  prism  the  ray  of  light  refracted 
by  it  is  always  turned  toward  its  back,  or 
thicker  part  (whether  that  be  turned  upward, 
downward,  or  to  either  side),  it  follows  that 
when  parallel  rays  fall  upon  a  double-convex 
^jff^  lens,  or  two  prisms  united  at  their  bases,  they 

will  converge  to  a  point. 

what  is  the  The  point  where  parallel  rays  of  light  fall- 
Sfincaipacofnvex  in8  uPon  one  s^6  °^  a  convex  lens  unite  by 
lens?  refraction  upon  the  opposite  side,  is  called  the 

principal  focus  of  a  lens. 

what  is  the        The  distance  from  the  middle  of  a  lens  to 
^Cflendsi8?tance    its  principal  focus,  is  called  the  focal  distance 
of  a  lens. 

This  in  a  single  convex  lens  is  equal  to  the  diameter  of  the  sphere  of  which 
the  lens  is  a  portion  ;  in  a  double-convex  lens  it  is  equal  to  the  radius,  or: 
semi-diameter  of  the  sphere  of  which  the  lens  is  a  portion. 

The  focal  distance  of  parallel  rays  falling  upon  a  convex  lens  is  repre- 
sented at  A,  Fig.  261.  If  the  rays  are  converging,  as  at  B,  they  will  come 
to  a  focus  sooner,  and  if  diverging,  as  at  C,  the  focus  will  be  further  from  the 
lens  than  for  parallel  rays. 

The  focus  of  a  convex  lens  may  be  easily  found  by  allowing  the  rays  of 
the  sun  to  fall  perpendicularly  upon  one  side  of  it,  while  a  sheet  of  paper  ia 

14* 


322 


WELLS'S  NATURAL   PHILOSOPHY. 


FIG.  261. 


on  what  prin- 


held  on  the  other.  A  bright  ring  of  light  will 
be  observed  on  the  paper,  diminishing  or  in- 
creasing in  size  according  to  the  distance  of 
the  paper  from  the  glass.  If  the  former  is  held 
in  such  a  manner  that  the  ring  of  light  is  re- 
duced to  a  dazzling  luminous  point,  as  is  rep- 
resented in  Pig.  262,  it  is  then  situated  in  the 
focus  of  the  glass. 

680.  From  their  prop- 
erty  of  converging  par- 
allel  rays   to   a   focus, 
convex  lenses,  like  con- 
cave mirrors,  may  be  used  for  the 

FIG-.  262.          production  of  high  temperatures,  by  con- 
centrating the  rays  of  the  sun. 

The  ordinary  burning,  or  sun-glass,  as  is  represented 
in  Fig.  262,  is  simply  a  double-convex  lens.  By  the 
employment  of  very  large  lenses,  a  degree  of  heat 
may  be  produced  far  exceeding  that  of  the  best  con- 
structed furnace.* 

In   the    employment   of   convex   lenses   as 
burning-glasses,  the  heat  concentrated  at  'the 
focus  is  to  the  common  heat  of  the   sun,  as 
the  area  of  the  surface  of  the  lens  is  to  the 
area  of  the  focus. 

Thus,  if  a  lens  four  inches  in  diameter  collects  the  sun's  rays  into  a  focus  at 
the  distance  of  twelve  inches,  the  focus  will  not  be  more  than  one  tenth  of  an 
inch  in  diameter;  its  surface,  therefore,  is  1,600  times  less  than  the  surface 
of  the  lens,  and  consequently  the  heat  will  be  1,600  times  greater  at  the  focus 
than  at  the  lens. 

681.  The  properties  of  a  concave  lens  are  greatly  dif- 
ferent from  those  of  a  convex  lens. 

Kays  falling  upon  a  concave  lens  are  so  re- 
fracted in  passing  through  it,  that  they  diverge 
on  emerging  from  the  lens,  as  though  they 
issued  from  a  focus  behind  it.  The  focus, 


How  does  the 
heat  at  the  fo- 
cus of  a  burn- 
ing-glass com- 
pare with  the 
heat  of  the  sun  ? 


What  is  the 
course  of  rays 
falling  upon  a 
double  con- 
cave lens  ? 


*  A  lens  of  this  character  was  constructed  many  years  since  in  England,  three  feet  in 
diameter,  with  a  focal  distance  of  six  feet  eight  inches.  Exposed  to  the  heat  concentrated 
in  the  focus  of  this  powerful  instrument,  the  metals  were  instantly  melted,  and  evtn  vol- 
atilized, while  quartz,  flint,  and  the  most  refractory  earthy  substances,  were  readily 
liquefied  and  caused  to  boil. 


REFRACTION  OF  LIGHT. 


323 


therefore,  of  a  concave  lens  is  not  real,  but  virtual,  as  is 
the  case  with  a*  convex  mirror. 

Thus,  in  Fig.  263, 

FIG.  263.  the  parallel  rays,  a  b 

cde,  etc.,  falling  upon 
the  double  concave 
lens,  L  L>,  are  so  re- 
fracted in  passing 
through  it,  that  they 
are  made  to  diverge, 
as  though  proceeding 
from  the  point  F,  be- 
hind the  lens. 

In  a  similar  man- 
ner convergent  rays  are  rendered  less  convergent,  or  even  parallel. 

DO  convex  682.  Images  are  formed  in  the  foci  of  con- 
1™^°™!  yex  lenses  in  the  same  way  as  in  the  foci  of 
tion.f  images?  concave  mirrors. 

Thus,  if  we  take  a  convex  lens  and  place  behind  it,  at  a  proper  distance,  a 
s'leet  of  paper,  there  will  be  depicted  upon  the  paper  beautifully  clear  and 
distinct  images  of  all  the  objects  in  front  of  4he  lens,  in  an  inverted  position. 
The  manner  in  which  they  are  formed  is  illustrated  in  Fig.  264. 

Thus,  let  A  B  F  r   264 

Describe      the  .  *IG-  ^^*- 

formation     of     represent  an  ob-  ^ 

images  by  the      ject  placed  be- 
convex  lens.  J 

fore    a  double 

convex  lens,  E  F.     The  rays 
proceeding  from  A,  the  top  of  -- 
the  object,  will  be  converged 
by  the  lens  and  brought  to  a 
focus   at  D,  where  they  will 
form  an  image;  the  rays  pro- 
ceeding from  B,  the  base  of  the  object,  will  also  be  converged  and  brought 
to  a  focus  at  C  ;  and  so  each  point  of  the  object,  A  B,  will  have  its  corre- 
sponding image  between  C  D.     In  this  way  a  complete  image  will  be  formed. 

The  image  formed  by  a  convex  lens  will  ap- 
pear inverted,  because  the  rays  of  light  from 
the    several   points   of  the  object  cross  each 
other  in  proceeding  to  the  corresponding  points 
of  the  image. 

Thus,  in  Fig.  264,  the  ray,  A  E,  proceeding  from  the  top  of  the  object  and 
falling  obliquely  upon  the  lens,  is  refracted  into  the  course  E  D,  and  in  like 
manner  the  ray  B  F  is  refracted  in  the  direction  F  C ;  and  as  these  rays  cross 


C -> 


Why  are  the 
images  formed 
by  convex 
lenses  invert- 
ed? 


324'  WELLS'S   NATURAL  PHILOSOPHY. 

each  other,  the  image  of  ttie  arrow  appears  inverted.  The  central  ray  of  light 
proceeding  from  the  object  in  the  direction  of  the  axis  G?  and  falling  perpen- 
dicularly upon  the  surface  of  the  lens,  undergoes  no  refraction,  but  continues 
on  in  a  direct  course. 

The  images  thus  formed  by  convex  lenses  maybe  rendered 
agerforSed  by  visible  by  being  received  upon  white  screens,  or  any  suitable 
convex  lenses  objects,  or  directly  by  the  eye,  when  placed  in  a  proper  posi- 
be^made  visi-  tioQ  to  receiye  the  ravs< 

When,  by  the  employment  of  the  convex  lens  as  a  burning- 
glass,  we  concentrate  on  any  suitable  surface,  the  sun's  rays  to  a  focus,  the  little 
luminous  spot,  or  circle  formed,  is  really  an  image,  or  picture  of  the  sun 
itself. 

why  are  con-  683.  Convex  lenses,  as  ordinarily  used,  are 
called  magnify  ing-glasses,  because  they  in- 
crease  the  apparent  size  of  the  objects  seen 
through  them. 

The  reason  of  this  is,  that  the  lens  so  alters, 
iens    by  refraction,  the  direction  of  the  rays  of  light 


proceeding  from  an  object,  that  they  enter  the 
eye  as  if  they  came  from  points  more  distant  from  each 
other  than  is  actually  the  case,  and  hence  the  object  ap- 
pears larger,  or  magnified. 

On  the  contrary,  the  concave  lens,  which 

Why    does    a  f  •  «• 

corcave    iena    produces  an  exactly  opposite  effect  upon  the 

diminish      the       ^  .  7i         •  n  •,  • 

apparent  size    rays  of  light,  causes  the  image  of  an  object 

of  an  object?  J  e       '  J 

seen  through  it  to  appear  smaller. 

On  the  same  principles  also,  concave  mirrors  magnify,  and  convex  mirrors 
dimmish  the  images  of  objects  reflected  from  their  surfaces. 

Hence  the  magnifying  or  diminishing  power 

What  is  said  of         „,  .  ,  •         /.  i      i  J 

the  magnifying   of  lenses  is  .not.  as  is  often  popularly  supposed, 

or   diminishing       ,  .  '  ..  X  „  J.       if  I 

power  of  lenses?  due  merely  to  the  peculiar  nature  of  the  glass  of 
which  they  are  made,  but  to  the  figure  of  their 
surfaces. 

The  double  convex  lens,  inclosed  in  a  convenient  setting  of  metal  or  horn, 
is  extensively  employed  by  watch-makers,  engravers,  etc.,  with  whom  it 
passes  under  the  general  name  of  lens. 

HOW  may  con-  684.  In  addition  to  the  effect  which  convex 
derXdeist8anBtrob-"  lenses  produce  by  magnifying  the  images  of 
jects  visible?  objects,  they  are  also  capable  of  rendering 
distant  objects  visible  which  would  be  invisible  to  the 


THE   ANALYSIS   OF   LIGHT. 


325 


Explain  more 
fully  the  action 
of  the  convex 
lens  in  this  re- 
spect? 


Baked  eye,  by  causing  a .  greater  number  of  rays  of  light 
proceeding  from  them  to  enter  the  eye. 

The  light  which  produces  vision,  as  will  bo  more  fully  ex- 
plained hereafter,  enters  the  eye  through  a  circular  opening 
called  the  pupil,  which  is  the  black  circular  spot  surrounded 
by  a  colored  ring,  appearing  in  the  center  of  the  front  of  tho 
eye.  Now,  as  the  rays  of  light  proceeding  from  an  object 
diverge  or  spread  out  in  every  direction,  the  number  which  will  enter  the  eyo 
will  be  limited  by  the  size  of  the  pupil.  At  a  great  distance  from  an  object, 
as  will  be  seen  in  Fig.  265,  few  rays  wiil  enter  the  eye;  but  if,  as  in  Fig.  266, 
we  place  before  the  eye  a  convex  lens  of  moderate  size,  a  large  number  of 
the  diverging  rays  will  be  collected  and  concentrated  into  a  single  point  or 
focus  behind  it,  and  thus  afford  to  the  eye  occupying  a  proper  position  suffi- 
cient light  to  enable  it  to  see  the  distant  object  distinctly. 

FIG.  265. 


FIG.  266. 


In  like  manner  a  concave  mirror,  by  causing  divergent  rays  which  fall 
upon  the  surface  to  become  convergent,  may  be  used  to  produce  the  same  ef- 
fect, as  is  shown  in  Fig.  267. 

FIG.  267. 


SECTION    I  I.I. 

THE     ANALYSIS     OF     LIGHT. 

685.  It  has,  up  to  this  point,  been  assumed,  that  light  is  a  simple  substance, 
and  that  all  its  rays,  or  parts,  are  refracted  in  precisely  the  same  manner,  and 
therefore  suffer  the  same  changes  when  acted  upon  by  transparent  media. 
This,  however,  is  not  its  constitution. 


326  WELLS'S  NATtJRAL  PHILOSOPHY. 

mat  is  the        White  light,  as  emitted  from  the  sun,  or 
whfteifgS  °f    from  any  luminous  body,  is  composed  of  seven 
different  kinds  of  light,  viz.,  red,  orange,  yel- 
low, green,  blue,  indigo,  and  violet. 

mat  is  the  The  seven  different  kinds  of  light  produce 
originofcoior?  geven  Different  colors,  viz.,  red,  orange,  yellow, 
green,  blue,  indigo,  and  violet.  These  seven  colors  are 
called  primary  colors,  since  by  the  union  or  mixture  of 
some  two  or  more  of  them,  all  other  colors,  or  varieties  of 
color  are  produced. 

HOW  is  light         The  separation  of  white  light  into  its  sev- 
eral parts  is  effected  by  means  of  a  prism. 
When  a  ray  of  white  light  is  made  to  pass  through  a 
prism,  each  of  the  seven  rays  of  which  it  is  composed 
are  refracted,  or  bent  out  of  their  course  differently,  and 
form  on  an  opposite  screen  or  wall  an  image  composed  of 
bands  of  the  seven  different  colors. 

what  is  the        686.  The  image  formed  by  a  ray  of  white 
spectrum?      Jjght  passing  through  a  prism,  is  called  the 
Solar,  or  Prismatic  Spectrum. 

FIG.  268. 


The  separation  of  a  ray  of  solar  light  into  different  colored  rays,  by  refrac- 
tion, ia  represented  in  Fig.  268*  A  ray  of  light,  S  A,  is  admitted  through, 
an  aperture  in  a  shutter  into  a  darkened  chamber,  and  caused  to  fall  on  a 
prism,  P.  The  ray  thus  entering  would,  if  allowed  to  pass  unobstmctedly,  Imve 
moved  in  a  straight  line  to  the  point  K,  on  the  floor  of  the  room,  and  there 


THE  ANALYSIS  OF  LIGHT.  327 

formed  a  circular  disc  of  white  light ;  but  by  the  interposition  of  the  prism 
the  ray  spreads  out  in  a  fan-shape,  and  forms  an  oblong  colored  image  on  the 
opposite  wall.  This  image,  called  the  solar  spectrum,  is  divided  horizontally 
into  seven  colored  spaces,  or  bands,  of  unequal  extent,  which  succeed  each 
other  in  an  invariable  order,  viz.,  red,  orange,  yellow,  green,  blue,  indigo,  violet. 

upon  what  does  The  separation  of  the  seven  different  rays 
ofVmfTiight  composing  white  light  from  one  another,  de- 
depend?  pends  entirely  upon  a  difference  in  their  re- 
frangiljility  in  passing  through  the  prism  ;  those*  which 
are  refracted  the  least  falling  upon  the  lowest  part  of  the 
screen,  and  those  which  are  refracted  the  most  upon  the 
upper  part. 

Thus  Ihe  red  rays,  which  are  the  least  refracted,  or  the  least  turned  from 
their  course  by  the  prism,  always  occur  at  the  bottom  of  the  spectrum,  while 
the  violet,  which  is  the  most  refracted,  occurs  at  the  top ;  the  remaining  colors 
being  arranged  in  the  intermediate  space  in  the  order  of  their  refrangibility. 

what  additional  The  seven  different  rays  of  light,  when  once 
of  The  comepo!i!  separated  and  refracted  by  a  prism,  are  not 
light  ?°f  white  capable  of  being  further  analyzed  by  refraction  ; 
but  if  by  means  of  a  convex  lens  they  are  col- 
lected together  and  converged  to  a  focus,  they  will  form 
white  light. 

If  the  spectrum  formed  by  a  prism  of  glass  be  divided  into  three  hundred 
and  sixty  parts,  it  is  found  that  the  red  ray,  or  color,  occupies  forty-five  of 
those  parts,  the  orange  twenty-seven,  the  yellow  forty-eight,  the  green  sixty, 
the  blue  sixty,  the  indigo  forty,  and  the  violet  eighty. 

If  we  take  a  circle  of  paper  and  paint  upon  it  in  divisions  of  proportionate 
size  the  seven  colors  of  the  spectrum,  and  then  cause  it  to  rotate  rapidly  about 
a  center,  the  colors  by  combination  will  impart  to  it  a  white  appearance.* 
From  this  and  other  experiments,  therefore,  it  is  inferred  that  light  which  we 
call  colorless,  or  white  (as  that  coming  immediately  from  the  sun),  really  con- 
tains light  of  all  possible  colors  so  mixed  as  to  neutralize  each  other. 

687.  The  separation  of  the  different  rays  of  light  which 
takes  place  in  their  passage  through  a  prism,  is  designated 
by  the  term  Dispersion. 

Explain    what  The  order  of"  refrangibility  of  the  seven  different  rays  of 

is  meant  by  the  light,  or  the  arrangement  of  the  seven  colors  in  the  spec- 

er^different  trum»  is  always  the  same  and  invariable,  whatever  way  the 

substances.  prism  may  be  turned ;  the  lower  end  of  the  spectrum  being 

*  It  is  very  common  to  find  it  stated  in  books  of  science  that  by  mixing  powders  of  the 
seven  different  colors  together  a  white,  or  grayish-white  compound  may  be  produced. 
The  experiment,  is  not,  however,  satisfactory. 


328 


WELLS'S  NATURAL  PHILOSOPHY. 


red,  which  passes  upward  into  orange,  then  into  yellow,  then  green,  blue, 
indigc,  and  violet,  which  is  at  the  upper  end. 

Dissimilar  substances,  however,  produce  spectra  of  different  lengths,  on  ac- 
count of  a  difference  in  their  refractive  properties.  Thus  a  ray  of  light  tra- 
versing a  prism  of  flint-glass,  will  have  its  red  aud  violet  colors  separated  on 
a  screen  twice  as  widely  as  those  of  a  ray  passing  through  a  similar  prism 
pf  crown-glass.  This  difference  is  expressed  by  saying  that  the  dispersive 
power  of  the  two  substances  is  different,  or  that  flint-glass  has  twice  the  dis- 
persive power  of  crown-glass. 

Why  will*  not  ^  a  lens  may  be  considered  as  a  modification^  of  the 
an  ordinary  prism,  it  follows  that  when  light  is  refracted  through  a  lens, 
perfect  image?  ifc  is  seParated  into  tne  different  colors,  precisely  as  by  a 
prism  ;  and  as  every  ray  contained  in  white  light  is  refracted 
differently,  every  lens,  of  whatever  substance  made,  will  have  a  different  focus 
for  every  different  color.  The  images,  therefore,  of  such  lenses  will  be  more 
or  less  indistinct,  and  bordered  with  colored  edges.  This  imperfection  is 
termed  chromatic  aberration. 

For  this  reason  the  focus  of  a  burning-glass,  which  is  an  optical  image  of 
the  sun,  is  never  perfectly  distinct,  but  always  confused  by  a  red,  or  blue  bor- 
der, since  the  various-colored  rays  of  which  sunlight  is  composed,  can  not 
all  be  brought  to  the  same  focus  at  once.  In  a  like  manner,  if  we  point  a 
common  telescope  at  a  blue  and  red  hand-bill  at  a  short  distance,  we  shah" 
have  to  draw  out  the  tube  of  the  instrument  to  a  greater  length  in  order  to 
read  the  red  than  the  blue  letters. 

These  fringes  of  color  are  a  most  serious  obstacle  to  the 
Explain       the 
construction  of     perfection  of  optical  instruments,    especially  in  astronomical 


FIG.  268. 


telescopes,  where  great  nicety  of  observation  is  required  ;  and 
to  prepare  a  lens  in  such  a  way  that  it  would  refract  light 
without  at  the  same  time  dispersing  it  into  colors,  was  long  considered  an  im- 
possibility. 

The  discovery  was,  however,  made  by  Mr.  Dollond, 
an  Englishman,  that  by  combining  two  lenses,  formed 
of  materials  which  refract  light  differently,  the  one 
might  be  made  to  counteract  the  effects  of  the  other  ;  on 
the  same  principle  as  by  combining  two  metals  together 
which  expand  unequally,  we  may  construct  a  pendu- 
lum whose  length  never  varies. 

Such  a  combination  is  represented  in  Fig.  268,  where 
a  convex,  lens  of  crown  glass  is  united  with  a  concave 
lens  of  flint  glass,  so  as  to  destroy  each  the  dispersive 
power  of  th^other,  while  at  the  same  time  the  refract- 
ing, or  converging  power  of  the  convex  lens  is  pre- 
served. A  lens  of  this  character  is  called  Achro- 
matic,* since  it  produces  images  in  their  natural 
colors. 
"  Achromatic,  from  a,  not,  and  x/>w/ia,  color. 


THE   ANALYSIS   OF   LIGHT.  329 

Lenses  are  also  subject  to  another  imperfec- 
aberration?  ^jon^  w|jich  [s  called  spherical  aberration.  This 
arises  from  the  fact  that  the  curved  surface  of  a  lens  is  at 
unequal  distances  from  the  object  and  from  the  screen 
which  receives  the  image  formed  at  its  focus  ;  and  hence, 
if  one  point  of  the  image  is  perfect,  another  point  is  less 
so,  owing  to  a  difference  in  the  convergence  of  the  rays 
coming  from  the  center  and  the  edges  of  the  lens. 

Thus,  if  the  image  is  received  on  a  screen  of  ground  glass,  it  will  be  found 
that  when  the  picture  is  well  defined  at  the  center,  it  will  be  indistinct  at  the 
edges ;  but  by  bringing  the  lens  nearer  the  screen,  the  edges  of  the  imago 
will  be  more  sharply  defined,  but  the  middle  is  indistinct.  To  make  the  im- 
age perfect,  therefore,  the  marginal  portions  of  the  lens  should  be  covered  with 
a  circlet  of  paper,  so  as  to  permit  those  rays  only  to  pass  which  lie  neat  the 
axis  of  the  lens.  This  plan,  however,  impairs  the  brightness  of  the  image. 

"When  the  image  formed  by  the  lens  is  small,  the  effect  of  spherical  aberration 
is  scarcely  noticed,  and  by  combination  of  lenses  of  different  refractive  powers, 
it  may  be  almost  entirely  overcome. 

688.  The  various  rays  composing  solar  light 

Are  all  the  rays  ,.  11       i          •  -  ,  i      f    • 

of  light  equally    are  not  all  equally  luminous,  that  is  to  say, 
they  do  not  appear  to  the  eye  equally  brilliant. 
The  color  most  visible  to  the  human  eye  is  yellow. 

The  luminous  intensity  of  the  different  colored  rays  of  light  may  be  ex- 
pressed numerically  as  follows: — Red,  94;  orange,  640;  yellow,  1,000; 
green,  480;  blue,  170;  indigo,  31;  violet,  6.* 

689.  According  to  some  authorities,  white  solar  light 
consists  of  only  three  colors — red,  yellow  and  blue,  which, 
by  combining,  produce  the  other  four  colors,  orange, 
green,  indigo  and  violet. 

whatareBome-       Eed>  yellow>  an<*  Wiie,  are,  therefore,  some- 
times called  the   times  called  the  simple  colors. 

simple  colors? 

Thus,  by  the  union  of  red  and  yellow,  we  may  produce 
orange ;  by  yellow  and  blue,  green ;  by  blue  and  red,  violet ;  indigo  being 
considered  as  merely  a  shade  of  blue.  .Red,  yellow,  and  blue,  on  the  contrary, 
can  not  be  produced  by  the  mingling  of  any  two  other  colors. 

When  blue  and  yellow  powders  are  mixed  together,  blue  and  yellow  rays 
are  reflected  to  the  eye  from  the  minute  particles,  but  the  two  colors  are  so 

*  It  would  appear,  from  numerous  observations,  that  soldiers  are  shot  during  battle 
according  to  the  color  of  their  dress  in  the  following  proportion : — red,  12 ;  dark  green,  7 ; 
brown,  6 ;  bluish  gray,  5.  Red  is  therefore  the  most  fatal  color,  and  a  light  gray  the 
least  so. 


330  WELLS'S  NATURAL  PHILOSOPHY. 

mingled  that  the  eye  only  notices  the  combined  effect,  which  is  green.  If  we 
now  examine  the  same  mixture  with  a  microscope,  the  blue  and  yellow  par- 
ticles will  be  seen  separately,  and  the  green  color  will  disappear. 

690.    The   natural   color  which   an   object 

Why   do   nat-  ...  " 

urai    objects     exhibits  when  exposed  to  the  light,  depends 

exhibit  colors  ?  *T 

upon  the  nature  and  arrangement  of  the  par- 
ticles of  matter  of  which  it  is  composed,  and  is  not  the  re- 
sult of  any  quality  inherent  in  the  object  itself. 

Bodies  which  naturally  exhibit  color  have,  by  reason  of 
a  certain  peculiar  arrangement  of  their  surfaces,  or  mole- 
cular structure,  a  greater  preference  for  some  qualities  of 
light  than  for  others.  If  the  body  is  not  transparent,  it 
will  reflect  certain  rays  of  light  from  its  surface,  and  ap- 
pear of  the  color  of  the  light  it  reflects  ;  if  the  body  is 
transparent,  it  will  allow  only  certain  rays  to  pass  through 
its  structure,  and  will  consequently  appear  of  the  color  of 
the  light  it  transmits. 

Thus  a  red  body  appears  red  because  it  reflects  or  transmits  the  red  ray  of 
solar  light  to  the  eye ;  and  a  yellow  body  appears  yellow  because  yellow 
light  is  reflected  or  transmitted  by  its  surface  or  structure  more  powerfully 
than  light  of  any  other  color;  and  so  on  through  all  the  colors. 

It  is  not,  however,  to  be  understood  that  colored  bodies  reflect  or  transmit 
only  pure  rays  of  one  color,  and  perfectly  absorb  all  others ;  on  the  contrary, 
it  has  been  found  that  a  colored  body  reflects,  in  great  abundance,  those  rays 
of  light  which  determine  its  particular  color,  and  also  the  other  rays  which 
make  up  white  light  in  a  greater  or  less  degree,  in  proportion  as  they  more 
or  less  resemble  its  color  in  the  order  of  their  refrangibility. 
wh  '  b  d  Some  substances  have  no  preference  for  any  one  quality  of 

coiortes8s%hen     -light  more  than   another,  but   reflect   or   absorb   them  all 
when'black?d     e<lually »  such  are  called  neutral,  or  colorless  bodies.     Those 
substances  which  reflect  all  the  rays  of  light  which  fall  upon 
them  appear  white  ;  those  which  absorb  all  the  rays  appear  black. 

In  the  dark  there  is  no  color,  because  there  is  no  light  to  be  absorbed  or 
reflected,  and  therefore  none  to  be  decomposed.  • 

A  glass  is  called  red  because  it  allows  the  red  rays  of  light  to  penetrate 
through  a  greater  thickness  of  its  substance  than  the  other  rays ;  but  at  a  cer- 
tain thickness,  even  the  red  rays  would  be  absorbed  like  the  rest,  and  we 
should  call  the  glass  black. 

No  body,  unless  self-luminous,  can  appear  of  a  color  not  existing  in  the 
light  which  it  receives.  This  may  be  proved  by  holding  a  colored  body  in  a 
ray  of  light  which  has  been  refracted  by  a  prism,  when  the  body  will  appear 
of  the  color  of  the  ray  in  which  it  is  placed;  for  since  it  receives  but  one  col- 
ored ray,  it  can  reflect  no  other. 


THE  ANALYSIS   OF  LIGHT.  331 

May  the  color  691.  By  changing  the  structure  or  molecu- 
cLnbg°<?die8  by  lar  arrangement  of  a  body,  the  color  which  it 
moaSartheir  exhibits  may  be  often  changed  also.  v  ;' 

Illustrations  of  this  principle  are  frequently  seen  in  chem- 
ical compounds.  The  iodide  of  mercury  is  a  beautiful  scarlet  compound,  which, 
when  gently  heated,  becomes  a  bright  yellow,  and  so  remains  when  undis- 
turbed. If,  however,  it  is  touched,  or 'scratched  with  a  hard  substance,  as 
with  the  point  of  a  pin,  its  particles  turn  over,  or  readjust  themselves,  and 
resume  their  original  red  color.  Chameleon  mineral  is  a  solid  substance  pro- 
duced by  fusing  manganese  with  potash  ;  when  dissolved  in  water,  it  changes, 
according  to  the  amount  of  dilution,  from  green  to  blue  and  purple.  Indigo 
also,  spread  on  paper  and  exposed  to  heat,  becomes  red. 

692.  Some  bodies  have  the  power  of  reflecting  from  their 
surfaces  one  color  while  they  transmit  another.  • 

This  is  the  case  with  the  precious  opal.  A  solution  of  quinine  in  water 
containing  a  little  sulphuric  acid,  is  colorless  and  transparent  to  the  eye  look- 
ing through  it,  but  by  looking  at  it,  it  appears  intensely  blue.  An  oil  ob- 
tained in  the  distillation  of  resin  transmits  yellow  light,  but  reflects  violet 
light.  Smoke  reflects  blue  light,  but  transmits  red  light.  These  phenomena 
result  from  a  peculiar  action  of  the  surface  or  outer  layer  of  the  substance 
of  the  body  on  some  of  the  rays  of  light  entering  it,  and  have  received  the 
name  of  epipolic,  or  surface  dispersion. 

Deepness  of  color  proceeds  from  a  deficiency,  rather  than  from  an  abund- 
ance of  reflected  rays :  thus,  if  a  body  reflects  only  a  few  of  the  red  rays,  it 
will  appear  of  a  dark  red  color.  When  a  great  number  of  rays  are  reflected, 
the  color  will  appear  bright  and  intense. 

If  the  objects  of  the  material  world  had  been  illuminated  only  with  white 
light,  all  the  particles  of  which  possessed  the  same  degree  of  refrangibility, 
and  were  equally  acted  upon  by  all  substances,  the  general  appearance  of 
nature  would  have  been  dull,  and  all  the  combinations  .of  external  objects, 
and  all  the  features  of  the  human  countenance  would  have  exhibited  no  other 
variety  than  that  which  they  posses  in  a  pencil  sketch  or  India-ink  drawing. 

what  are  com-        693.  Any  two  colors  which  are  able,  by  com- 
/Dining,  to  produce   white  light,   are  termed 
/Vcomplementary  colors.  ^.t,. 

Each  color  of  the  solar  ray  has  its  complementary  color, 
for  if  it  be  not  white,  it  is  deficient  in  certain  rays  that 
would  aid  in  producing  white.  And  these  absent  rays 
compose  its  complementary  color. 

The  relative  position  of  complementary  colors  in  the  prismatic  spectrum  may 
be  determined  as  follows:  Thus,  if  we  take  half  the  length  of  a  spectrum  by 
a  pair  of  compasses,  and  fix  one  leg  on  any  color,  the  other  leg  will  fall  upon 


332  WELLS'S  NATURAL  PHILOSOPHY. 

its  complementary  color,  or  upon  the  one  which  added  to  the  first  will  pro- 
duce white  light.  The  complementary  color  of  red  is  bluish  green ;  of 
orange  is  blue ;  of  yellow  is  mdigo ;  of  green  is  reddish  violet ;  of  blue  is 
orange  red ;  of  indigo  is  orange  yellow ;  of  violet  is  yellow  green ;  of  black  is 
.white ;  of  white  is  black. 

Complementary  colors  may  be  seen  by  fixing  the  eye  steadily  upon  any 
colored  object,  such  as  a  wafer  upon  a  sheet  of  white  paper.  A  ring  of  col- 
ored light  will  play  round  the  wafer,  and  this  ring  will  be  complementary  to 
the  color  of  the  wafer.  A  red  wafer  will  give  a  green  ring,  a  blue  wafer  an 
orange- colored  ring,  and  so  on.  Or  if,,  after  having  regarded  the  colored  wafer 
steadily  for  a  few  moments,  the  eye  be  closed,  or  turned  away,  it  will  retain 
the  impression  of  the  wafer,  not  in  its  own,  but  in  its  complementary  color  j 
thus  a  red  wafer  will  give  a  green  ray,  and  so  on. 

In  like  manner,  if  we  look  at  a  red  hot  fire  for  a  few  minutes,  every  object 
as  we  turn  away  appears  tinged  with  bluish  green. 

The  art  of  harmonizing  and  contrasting  colors  is  intimately  connected  with 
the  principles  of  complementary  colors. 

HOW  do  colors  Every  color  placed  beside  another  color  is 
changed,  and  appears  differently  from  what  it 
does  when  seen  alone  ;  it  equally  modifies, 

moreover,  the  color  with  which  it  is  in  proximity. 

As  a  general  rule,  two  colors  will  appear  to  the  best 

advantage  when  one  ig  complementary  to  the  other. 

Thus,  if  a  dress  is  composed  of  cloths  of  two  colors,  the  one  complementary 
to  the  other,  as  red  and  green,  orange  and  blue,  yellow  and  violet,  they  will 
mutually  heighten  the  effect  of  each,  and  make  each  portion  appear  to  the 
best  advantage.  For  this  reason,  a  dress  composed  of  cloths  of  different 
colors,  looks  well  for  a  much  longer  time,  although  worn,  than  one  of  a  single 
color,  the  character  of  the  fabric  being  the  same  in  both  instances. 

A  suit  of  clothes  of  one  color  can  be  worn  to  advantage  only  when  it  is 
new,  because  as  soon  as  one  portion  of  the  suit  loses  its  freshness  from  hav- 
ing been  worn  longer  than  another,  the  difference  will  increase  by  contrast. 
Thus  a  pair  of  new  black  pantaloons  worn  with  a  vest  of  the  same  color, 
which  is  old  and  rusty,  will  make  the  tinge  of  the  latter  appear  more  con- 
spicuous, and  at  the  same  time  the  black  of  the  pants  will  appear  more 
brilliant.  "White  and  other  light-colored  pantaloons  would  produce  a  contrary 
effect 

In  printing  letters  on  colored  paper,  the  best  effect  will  be  produced  when 
the  color  of  the  paper  is  complementary  to  the  ink ;  blue  should  be  put  upon 
orange,  and  red  upon  green. 

Stains  will  be  less  visible  on  a  dress  of  different  colors  than  on  one  com- 
posed of  only  a  single  color,  since  there  exists  in  general  a  greater  contrast 
among  the  various  parts  of  the  first-named  dress,  than  between  the  stain  and 
the  adjacent  part,  and  this  difference  renders  the  stain  less  apparent  to  the  eye. 


THE  ANALYSIS  OF   LIGHT.  333 

In  the  grouping  of  flowers  in  gardens,  and  in  .the  preparation  of  bouquets, 
the  most  pleasing  effects  will  be  produced  by  placing  the  blue  flowers  next 
to  the  orange,  and  the  violet  next  to  the  yellow.  White,  red,  and  pink 
flowers  are  never  seen  to  greater  advantage  than  when  surrounded  with  green 
leaves,  or  white  flowers ;  on  the  other  hand,  we  should  always  separate  pink 
flowers  from  those  that  are  either  scarlet  or  crimson ;  orange,  from  orange- 
yellow  flowers ;  yellow  flowers  from  greenish -yellow  flowers ;  blue  from  violet- 
blue,  red  from  orange,,  pink  from  violet. 

By  grouping  colors  together  which  are  not  complementary,  or  which  do  not 
rightly  contrast  with  each  other,  we  produce  a  discordant  effect  upon  the  eye, 
analogous  to  the  discord  which  is  produced  upon  the  ear  by  instruments  out  of 
tune.  It  is  always  necessary  that,  if  one  part  of  the  dress  be  highly  ornamented, 
or  consists  of  various  colors,  a  portion  should  be  plain,  to  give  repose  to  the  eye. 

Black  being  the  complementary  color  of  white,  the  effect  of  black  drapery 
upon  the  color  of  the  skin  or  face  is  to  make  it  appear  pale,  or  whiter  than  it 
usually  is. 

The  optical  effect  of  dark  and  black  dresses  is  to  make  the  figure  appear 
smaller ;  hence  it  is  a  suitable  color  for  stout  persons.  On  the  contrary,  white 
and  light-colored  dresses  make  persons  appear  larger.  Large  patterns  or  de- 
signs upon  dress,  make  the  figure  appear  shorter :  longitudinal  stripes,  if  not 
too  wide,  add  to  the  height  of  the  figure ;  horizontal  stripes  have  a  contrary 
tendency,  and  are  very  ungraceful.* 

matisaRain-        694.  The  Kainbow  is  a  semicircular  band 
or  arch,  composed  of  the  seven  different  colors, 
generally  exhibited  upon  the  alouds  during  the  occurrence 
of  rain  in  sunshine. 

HOW  is  a  ram-  The  rainbow  is  produced  by  the  refraction 
bow  produced?  and  reflection  Of  tne  solar  rays  in  the  drops  of 
falling  rain. 

'  The  following  curious  facts  are  known  to  persons  employed  in  trade:— "When  a  pur- 
chaser has  for  a  considerable  time  looked  at  a  yellow  fabric,  and  is  then  shown  orange  or 
scarlet  stuffs,  he  considers  them  to  be  amaranth-red,  or  crimson,  for  there  is  a  tendency 
in  the  eye,  excited  by  yellow,  to  see  violet,  whence  afl  the  yellow  of  the  scarlet  or  orange 
cloth  disappears,  and  the  eye  sees  red,  or  red  tinged  with,  scarlet.  Again,  if  there  are 
presented  to  a  buyer,  one  after  another,  fourteen  piece*  of  rod  cloth,  he  will  consider  the 
last  six  or  seven  less  beautiful  than  those  first  seen,  aJthengh  the  pieces  be  identically  the 
same.  Now  what  is  the  cause  of  this  error  in  judgment?  It  is  that  the  eyes  having 
seen  seven  or  eight  red  pieces  in  succession,  are  in  the  same  condition  as  if  they  had 
regarded  fixedly  during  the  same  period  of  time  a  single  pioce  of  red  cloth;  they  have 
then  a  tendency  to  see  the  complementary  color  of  rod,  that  is  to  say,  green.  This  tend- 
ency goes,  of  necessity,  to  enfeeble  the  brilliancy  of  ftie  red  of  the  pieces  seen  later.  In 
order  that  the  merchant  may  not  be  the  sufferer  by  this  failing  of  the  eyes  of  his  cus- 
tomer, he  must  take  care  after  having  shown  the  latter  seven  pieces  of  red,  to  present  to 
him  some  pieces  of  green  cloth,  to  restore  the  eyes  to  their  natural  state.  If  the  sight  of 
the  green  be  sufficiently  prolonged  to  exceed  the  normal  state,  the  eyes  will  acquire  a 
tendency  to  see  red ;  then  the  last  seven  pieces  w2U  appear  more  beautiful  than  the 
others."— Ckevreul  on  Color. 


334 


WELLS'S  NATURAL  PHILOSOPHY. 


What  experi- 
ments prove 
the  decomposi- 
tion of  light  by 
drops  of  wa- 
ter? 


695.  Rainbows  are  also  formed  when  the  sun  shines  upon  drops  of  water 
Ming  hi  quantity  from  fountains,  waterfalls,  paddle-wheels,  etc. 

That  the  rainbow  results  from  the  decomposition  of  the  solar 
rays  by  drops  of  water,  may  be  proved  by  the  following  sim- 
ple experiment: — If  we  take  a  glass  globe  filled  with  water, 
and  suspend  it  at  a  certain  height  in  the  solar  rays  above  the 
eye,  a  spectator  standing  with  his  back  to  the  sun  will  see 
the  refraction  and  reflection  of  red  light;  if,  then,  the  globe  be  lowered 
slowly,  the  observer  retaining  his  position,  the  red  light  will  be  replaced 
by  orange,  and  this  in  its  turn  by  yellow,  and  so  on,  the  globe  at  dif- 
ferent heights  presenting  to  the  eye  the  seven  primitive  colors  in  succession. 
If  now,  in  the  place  of  the  globe  occupying  different  positions,  we  sub- 
stitute drops  of  water,  we  have  a  ready  explanation  of  the  phenomena  of 
the  rainbow. 

Drops  of  rain,  suspended  to  grass  or  bushes,  may  be  frequently  found  to 
appear  to  the  eye  of  a  bright  red;  and  by  slightly  changing  the  position  of  the 
eye,  the  colors  of  the  drop  may  be  made  to  appear  successively  yellow,  green, 
blue,  violet,  and  also  colorless.  This  also  proves  that  rays  of  light,  falling  in 
certain  directions  upon  drops  of  water,  are  refracted  thereby  and  decomposed 
into  colored  rays  that  become  visible  to  the  eye  when  it  is  situated  in  the 
proper  direction. 

Jj  IGr.   *iu«J.  - 

formation  of  the  rain- 
bow may  be  further 
illustrated  by  Fig.  269. 
Let  A  B  and  C  be  three 
drops  of  rain;  S  A, 
S  B,  and  S  C,  three 
rays  of  the  sun.  The 
ray  S  A,  by  refraction, 
is  divided  into  three 
colors ;  the  blue  and 
yellow  are  bent  above 
the  eye,  D,  and  the 
red  enters  it. 

The  ray,  S  B,  is  di- 
vided into  three  col- 
ors ;  the  blue  is  bent  above  the  eye,  and  the  red  falls  below  the  eye  D,  but 
the  yellow  enters  it. 

The  ray,  S  C,  is  also  divided  into  three  colors.  The  blue  (which  is 
bent  most)  enters  the  eye,  and  the  other  two  fall  below  it.  Thus  the 
eye  sees  the  blue  of  C,  and  of  all  drops  in  the  position  of  C ;  the 
yellow  of  B,  and  of  all  drops  in  the  position  of  B;  and  the  red  of  A, 
and  of  all  drops  in  the  position  of  A.  The  same  may  be  also  inferred 
respecting  the  other  four  colors  of  the  spectrum;  and  thus  the  eye  sees 
a  rainbow. 


S 


THE  ANALYSIS  OF   LIGHT. 


335 


what  are  the         ^e  ra™\)ow  can  ^e  seen  outy  when  it  rains, 
conditions  nee-    an^  jn  that  point  of  the  heavens  which  is  op- 

essary  in  order  JT  •»• 

bowT  a  raia"     Posite  to  thQ  sun. 

Hence  a  rainbow  is  always  observed  to  be  situated  in  the 
west  in  the  morning,  and  in  the  east  in  the  afternoon. 

It  is  algo  necessary  for  the  production  of  a  rainbow 
that  the  height  of  the  sun  above  the  horizon  should  not 
exceed  forty-two  degrees. 

Hence  we  generally  observe  this  phenomenon  in  the  morning,  or  toward 
evening;  and  it  is  only  in- the  winter,  when  the  sun  stands  very  low,  that  tho 
rainbow  is  sometimes  seen  at  hours  approaching  noon. 

Is    the    game         As  tlie  rays  of  light  differ  greatlv  in  refrangibility,  only  a 
rainbow    seen     single  and  different-colored  ray  from  each  drop  will  reach  the 
eye  of  a  spectator ;  but  as  in  a  shower  there  is  a  succession 
of  drops  in  all  positions  relative  to  the  eye,  the  eye  is  en-, 


alike  by  all  per- 
sons? 


210. 


abled  to  receive  the  different-colored 

rays  refracted  at  different  inclina- 

tions.    This  is  clearly  illustrated  in 

Fig.   270,  in  which  S  represents 

rays  of  the  sun  falling  upon  suc- 

cessive drops,  R,  0,  Y,  G,  B,  I,  V  ; 

but  a  single   colored  ray,   and  a 

different  one  for  each  drop,  will 

reach  the  eye.     As  no  two  spec- 

tators can  occupy  exactly  the  same 

position,  no  two  can  see  the  same 

color  reflected  from  the  same  drop  ; 

and  consequently  no  two  persons  see  the  same  rainbow. 

In  the  formation  of  a  rainbow  each  colored  ray  reflected 
from  the  falling  dr°PS  of  rain,  enters  the  eye  at  a  different  inclin- 
ation or  angle.  But  the  several  positions  of  those  drops, 

which  alone  are  capable  of  reflecting  the  same  color  at  the  same  angle,  to 

the  eye  constitute  a  circle,  —  and  hence  the  bands  of  color  which  make  up  a 

rainbow,  appear  circular. 

what  are  pri-  Two  rainbows  are  not  unfrequently  observed 
at  the  same  time,  the  one  being  exterior  to, 
and  less  strongly  developed  than  the  other. 
The  inner  arch,  which  is  the  brightest,  is  called  the  pri- 
mary bow,  and  the  outer,  or  fainter  arch,  the  secondary 
bow.  The  order  of  colors  in  the  inner  bow  is  also  the  re- 
verse of  that  in  the  outer  bow. 


336 


WELLS'S   NATURAL   PHILOSOPHY. 


How 


FIG.  272. 


is   the        The  inner,  or  primary  rainbow,  which  is  the 
one  ordinarily  seen,  is  formed  by  two  refrac- 
tions of  the  solar  ray,  and  one  reflection,  the 
ray  of  light  entering  the  drops  FIG.  27 1. 

at  the  top,  and  being  reflected  to 
the  eye  from  the  bottom. 

Thus,  in  Fig.  271,  the  ray  S  A  of  the  pri- 
mary rainbow  strikes  the  drop  at  A,  is  re- 
fracted, or  bent  to  B,  the  back  part  of  tho 
inner  surface  of  the  drop ;  it  is  then  reflected 
to  C,  the  lower  part  of  the  drop,  when  it  is 
refracted  again,  and  so  bent  as  to  come  di- 
rectly to  the  eye  of  the  spectator. 

HOW  is  the  sec-        The  secondary,  or  outer  rainbow,  is  produced 

bowa!brmed1n~    ty  two  refractions  of  the  solar  ray,  and  two 

reflections,  the  ray  of  light  entering  the  drops 

at  the  bottom,  and  being  reflected  to  the  eye  from  the  top. 

Thus,  in  Fig.  272,  the  ray  S  B  of  the  sec- 
ondary bow  strikes  the  bottom  of  the  drop 
at  B,  is  refracted  to  A,  is  then  reflected  to 
C,  is  again  reflected  to  D,  when  it  is  again 
refracted  or  bent,  till  it  reaches  the  eye  of 
the  spectator. 

The  position  and  formation  of  the  primary 
and  secondary  rainbows  are  represented  in 
Fig.  273.  Thus,  in  the  formation  of  the  pri- 
mary bow,  the  ray  of  light  S  strikes  tho  drop 
n  at  a,  is  refracted  to  &,  reflected  to  g,  and 
leaving  the  drop  at  this  point,  is  refracted 

to  the  eye  of  the  spectator  at  0.  In  the  formation  of  the  secondary  bow, 
the  ray  S'  strikes  the  drop  p  at  the  bottom  at  the  point  t,  is  refracted  to  rf, 
reflected  to/  and  thence  to  e,  and  refracted  from  the  top  of  the  drop,  pro- 
ceeds to  the  eye  of  the  spectator  at  0. 

The  reason  the  outer  bow  is  paler  than  the  inner  is  because  it  is  formed  by 
rays  which  have  undergone  a  second  internal  reflection,  and  after  every  re- 
flection light  becomes  weaker. 

what   are  Halos  are  colored  rays  which  are  sometimes 

seen  surrounding  luminous  bodies,  especially 
the  sun  and  moon.  They  are  occasioned  by  the.  refraction: 
and  decomposition  of  light  by  particles  of  moisture,  or 
crystals  of  ice  floating  in  the  higher  regions  of  the  atmos- 
phere, and  are  never  seen  when  the  sky  is  perfectly  clear. 


THE   ANALYSIS   OF   LIGHT. 
FIG.  273, 


337 


The  production  of  halos  may  be  illustrated  experimentally,  by  crystallizing 
various  salts  upon  plates  of  glass,  and  looking  through  the  plates  at  the  sun, 
or  a  candle.  A  few  drops  of  a  saturated  solution  of  alum,  spread  over  a 
glass  so  as  to  crystallize  quickly,  will  cover  it  with  an  imperfect  crust  of  crys- 
tals, scarcely  visible  to  the  eye.  Upon  looking  at  a  luminous  body  through 
the  glass  plate,  with  the  smooth  side  next  the  eye,  three  fine  halos  will  be 
perceived  encircling  the  source  of  light 

The  fact  that  halos,  or  rings  round  the  moon,  are  more  frequently  observed 
than  solar  halos,  is  dependent  upon  the  circumstance  that  the  sun's  light  is 
too  intense  and  dazzling  to  allow  the  halo  to  be  recognized.  Halos  may  be 
observed  most  frequently  in  the  winter  season,  and  in  high  northern  latitudes. 

696.  The  beautiful  crimson  appearance  of 
e  clouds  after  sunset  in  the  western  horizon, 
sunriseloudand    *s  ^ue  *n  a  grea^  measure  to  the  fact  that  the 
red  rays  of  the  solar  light  are  less  refrangible 


what  is  the 


FIG.  274. 


than  any  of  the 
other  colored  rays, 
and  in  conse- 
quence of  this, 
they  are  not  bent 
out  of  their  course 
so  much  as  the 
blue  and  yellow 
rays,  and  are  the 
last  to  disappear. 
For  the  same  rea- 


333  WELLS'S  NATURAL  PHILOSOPHY. 

son  they  are  the  first  to  appear  in  the  morning  when  the 
sun  rises,  and  impart  to  the  morning  clouds  red  or  crim- 
son colors. 

Let  us  suppose,  as  in  Fig.  274,  a  ray  of  light  proceeding  from  the  sun,  S, 
to  enter  the  earth's  atmosphere  at  the  point  P.  The  red  rays,  which  com- 
pose in  part  the  solar  beam,  being  the  least  refrangible,  or  the  least  deviated 
from  their  course,  will  reach  the  eye  of  a  spectator  at  the  point  A ;  while 
the  yellow  and  blue  rays,  being  refracted  to  a  greater  degree,  will  reach  the 
surface  of  the  earth  at  the  intermediate  points  B  and  C.  They  will,  conse- 
quently, be  quite  invisible  from  the  point  A. 

The  red  and  golden  appearance  of  the  clouds  at  morning  and  evening  is 
also  due  in  part  to  the  fact,  that  aqueous  vapor  on  the  point  of  being  con- 
densed, only  allows  the  red  and  yellow  rays  of  light  to  pass  through  it.  For 
thi^  reason,  if  the  sun  be  viewed  through  a  column  of  steam  escaping  from 
a  boiler,  it  appears  of  a  deep  red,  or  crimson  color.  The  same  thing  may  be 
noticed  during  a  drought  in  summer,  when  the  air  is  filled  with  dry  exhala- 
tions. 

mat     ia  697.  The  irregular  brilliancy  of  the  stars, 

Twinkling?  known  as  twinkling,  is  supposed  to  be  due  to 
unequal  reflections  of  light  occasioned  by  inequalities  and 
undulations  in  the  atmosphere. 

now  is  color  698.  Light,  according  to  the  undulatory 
thfundtnatory  theory,  is  occasioned  by  the  vibrations  or  un- 
theory  of  light?  durations  of  a  certain  elastic  medium  diffused 
throughout  all  space,  called  ETHER.  Color,  according  to 
this  theory,  depends  on  the  number  of  vibrations  which 
are  made  in  a  certain  time  ;  those  vibrations  which  are  the 
most  rapid,  producing  upon  the  eye  the  sensation  of  violet, 
and  those  which  are  the  slowest,  the  sensation  of  red. 

The  analogy  between  sound  and  light,  according  to  the 
is^there*1^  undulatory  theory,  is  perfect,  even  in  its  minutest  circum- 
tween  color  and  stances.  When  a  certain  number  of  vibrations  of  a  musical 
music1?0469  °  chord  are  caused  in  a  given  time,  we  produce  a  required 
sound ;  as  the  vibrations  of  the  chord  vary  from  a  quick  to  a 
slow  rate,  we  produce  sounds  sharp  or  grave.  So  with  light ;  if  the  rate  at 
which  the  ray  undulates  is  altered,  a  different  sensation  is  made  upon  the 
organs  of  vision. 

The  number  of  aerial  vibrations  per  second  required  to  produce  any  particu- 
lar note  in  music  has  been  accurately  calculated,  and  it  is  also  known  that 
the  ear  is  able  to  detect  vibrations  producing  sound,  through  a  range  com- 
mencing with  15,  and  reaching  as  far  as  48,000  in  a  second.  So  also  in  the 
case  of  light,  the  frequency  of  vibrations  of  the  ether  required  for  the  produc- 


THE   ANALYSIS   OF   LIGHT.  339 

tion  of  any  particular  color  has  been  determined,  and  the  length  of  the  wavea 
corresponding  to  these  vibrations. 

what  relation  The  waves  requisite  to  produce  red  are  the 
fhests  wtv?-611  largest ;  orange  comes  next ;  then  yellow, 
brafionsaofdthe  green?  blue,  indigo,  and  violet,  succeed  each 
different  colors?  Other,  the  waves  of  each  being  less  than  the 
preceding.  The  rapidity  of  vibration  is  in  the  same  order, 
the  waves  producing  red  light  vibrating  with  the  least 
rapidity,  and  the  waves  producing  violet  with  the  greatest 
rapidity. 

To  produce  red  light  it  is  necessary  that  40,000  waves  or  undulations  should 
be  comprised  within  the  space  of  a  single  inch,  and  that  480  billions  of  vibra- 
tions should  be  executed  in  one  second  of  time ;  while,  for  the  production  of 
violet,  60,000  waves  within  an  inch,  and  720  billions  of  vibrations  per  second 
are  required.* 

699.  As  two  sets  of  sound-waves  or  vibra- 

Can  waves   of  .  .  . .  „  . 

light  be  made    tions  may  so  combine  as  to  modify  or  destroy 

to  interfere?  i        Ai  j    Al  I-    i  ±    i 

each  other,  and  thus  produce  partial  or  total 
silence,  so  two  waves  or  vibrations  of  light  may  be  made 
to  interfere  and  produce  various  colors,  or  entire  darkness. 

*  It  may  perhaps  be  asked,  with  something  of  incredulity,  how  such  a  result  could  pos- 
sibly have  been  arrived  at,  with  any  degree  of  scientific  accuracy.  The  problem,  how- 
ever, is  not  a  difficult  one. 

In  the  first  place,  Newton,  by  a  series  of  perfectly  satisfactory  and  beautiful  experi- 
ments, ascertained  the  number  of  waves  or  undulations  of  the  different  colored  rays 
comprised  within  the  space  of  an  inch. 

Let  us  now  suppose  an  object  of  any  particular  color,  a  red  star,  for  example,  to  be 
viewed  from  a  distance.  From  the  star  to  the  eye  there  proceeds  a  continuous  line  of 
waves ;  these  waves  enter  the  pupil,  and  impinge  upon  the  retina ;  for  each  wave  which 
thus  strikes  the  retina,  there  will  be  a  separate  pulsation  of  that  membrane.  Its  rate  of 
pulsation,  or  the  number  of  pulsations  which  it  makes  per  second,  will  therefore  be  known, 
if  we  can  ascertain  how  many  luminous  waves  enter  the  eye  per  second. 

It  has  been  already  shown  that  light  moves  at  the  rate  of  about  200,000  miles  per 
second ;  it  follows,  that  a  length  of  ray  amounting  to  200,000  miles  must  enter  the  pupil 
each  second ;  the  number  of  times,  therefore,  per  second,  which  the  retina  will  vibrate, 
will  be  the  same  as  the  number  of  the  luminous  waves  contained  in  a  ray  200,000  miles 
long. 

Let  us  take  the  case  of  red  light.  In  200,000  miles  there  are,  in  round  numbers, 
1,000,000,000  feet,  and  therefore  12,000,000,000  inches.  In  each  of  these  12,000,000,000  of 
inches  there  are  40,000  waves  of  red  light.  In  the  whole  length  of  the  ray,  therefore,  thero 
are  480,000,000,000,000  waves.  Since  this  ray,  however,  enters  the  eye  in  one  second, 
and  the  retina  must  pulsate  once  for  each  of  these  waves,  we  arrive  at  the  astounding 
conclusion,  that  when  we  behold  a  red  object,  the  membrane  of  the  eye  trembles  at  the 
rate  of  480,000,000,000,000  of  times  between  every  two  ticks  of  a  common  deck ! 

In  the  same  manner,  the  rate  of  pulsation  of  the  retina  corresponding  to  other  tints  of 
colors  is  determined ;  and  it  is  found  that  when  violet  is  perceived,  it  trembles  at  the  rate 
of  720,000,000,000,000  of  times  per  second.— Lctrdner. 


340 


WELLS'S   NATURAL   PHILOSOPHY. 


How  may  the 
interference  of 


HOTT  ia  color 
produced  by 
the  interfer- 


If  we  stand  at  the  junction  of  two  streams  of  water,  it  will 
be  noticed  that  when  the  waves  from  each  meet  in  the  same 
state  of  vibration,  the  resulting  wave  will  be  equal  to  the  two 
combined ;  if,  however,  one  wave  is  half  an  undulation  behind 
the  other,  the  crest  of  one  will  meet  the  hollow  of  the  other,  and  compara- 
tively smooth  water  will  be  the  result.  So  if  two  pencil  rays  of  light,  radiat- 
ing from  two  points,  reach  a  point  of  interference  at  the  same  degree  of  ele- 
vation, a  spot  of  double  the  luminous  intensity  of  either  will  be  produced ; 
but  if  one  is  hah0  a  vibration  behind  the  other,  the  result  will  be,  that  a  dark 
instead  of  a  light  spot  will  be  apparent. 

The  brilliant  tints  of  soap  bubbles,  and  thin 
plates  of  different  transparent  bodies,  are  ex- 
ence  of  ught?  ampies  Of  the  interference  of  light;  for  the 
undulations  reflected  from  the  first  surface  interfere  with 
those  reflected  from  the  second,  and  thus  produce  the 
various  colors. 

The  varying  play  of  colors  exhibited  by  films  of  oil  on  the  surface  of  water, 
and  the  iridescent  appearance  of  mother-of-pearl,  the  scales  of  fishes,  and  the 
wings  of  some  insects,  are  all  phenomena  resulting  from  the  interference  of  light. 

what  is  double  700.  Double  refraction  is  a  property  which 
refraction?  certain  transparent  substances  possess,  of 
causing  a  ray  of  light  in  passing  through  them  to  undergo 
two  refractions  ;  that  is,  the  single  ray  of  light  is  divided 
into  two  separate  rays. 

A  very  common  mineral  called  "  Iceland  spar," 
which  is  a  crystallized  form  of  carbonate  of  lime,  ia 
a  remarkable  example  of  a  body  possessing  double 
refracting  properties.  It  is  usually  transparent  and 
colorless,  and  its  crystals,  as  shown  in  Fig.  275,  have 
the  geometrical  form  of  a  rhomb,  or  rhomboid ;— this 
term  being  applied  to  a  solid  bounded  by  parallel 
faces,  inclined  to  each  other  at  an  angle  of  105°. 
manner  in  which  a  crystal  of 


FIG.  275. 


Illustrate  the 
phenomenon  of 
double  refrac- 
tion. 


The 

Iceland  spar  divides  a   ray  of  light  in- 
to   two    separate    portions    is    clearly 


FIG.  27G. 


represents  a  ray  of  light,  falling  upon  a  surface  of  a 
crystal  of  Iceland  spar,  A  D  E  C,  in  a  perpendicular  di- 
rection. Instead  of  passing  through  without  any  refrac- 
tion, as  it  would  in  case  it  had  fallen  perpendicularly  upon 
the  surface  of  glass,  the  ray  is  divided  into  two  separate 
rays,  the  one,  T  0,  being  in  the  direction  of  the  original 
ray,  and  the  other,  T  E,  being  bent  or  refracted.  The 
first  of  these  rays,  or  the  one  which  follows  the  ordinary 


THE   ANALYSIS   OF   LIGHT.  341 

law  of  refraction,  is  called  the  "  ordinary"  ray  ;  the  second,  which  follows  a 
different  law,  is  called  the  "  extraordinary"  ray. 

If  we  look  at  a  small  object,  as  a 
FIG.  277.  or  a  jjne)  through  a 


plate  of  glass,  it  appears  single  ;  but 
if  a  plate  of  Iceland  spar  be  sub- 
stituted, a  double  image  will  be  per- 
ceived, as  two  dots,  two  letters,  two 
lines,  etc.  This  result  of  double  re- 
fraction is  represented  in  Fig.  277. 
Crystals  of  many  other  substances, 
such  as  mica,  the  topaz,  gypsum,  etc., 
possess  the  property  of  double  refraction,  but  not  in  so  remarkable  a  degree 
as  Iceland  spar. 

In  all  these  crystals,  there  are  one  or  more  directions  along 
axe*ofa<ioubie  which  objects  when  viewed  through  them  appear  single; 
refraction?  these  directions  are  termed  the  lines,  or  axes  of  double  re- 
fraction. In  the  case  of  Iceland  spar,  there  is  one  axis  of  double  refraction, 
f.  e.,  one  direction  along  which  objects  when  viewed  appear  single  ;  this  is  in 
the  direction  of  the  line  A  B,  Fig.  275,  which  joins  the  two  obtuse  three- 
sided  angles.  If  the  summits  A  and  B  be  ground  down  and  polished,  no 
double  refraction  will  occur  in  looking  through  the  crystal  in  this  direction. 
To  what  is  the  That  tbe  phenomenon  of  double  refraction  is  due  entirely  to 
phenomenon  of  the  molecular  structure  of  the  medium  through  which  light 
P*8868'  is  proved  by  taking  a  cube  of  regularly  annealed  glass, 
which  produces  but  one  refracted  ray,  and  heating  it  unequally, 
by  subjecting  it  to  pressure  :  a  change  is  thereby  affected  in  the  arrangement 
of  its  parts,  and  double  refraction  takes  place. 

mat  in  poiar-  701.  When  a  ray  of  light  has  been  reflected 
ized  light?  £rom  tke  surface  of  a  body  under  certain 
special  conditions,  or  transmitted  through  certain  trans- 
parent crystals,  it  undergoes  a  remarkable  change  in  its 
properties,  so  that  it  is  no  longer  reflected  and  refracted 
as  before.  The  effect  thus  produced  upon  it  has  been 
called  polarization,  and  the  ray  or  rays  of  light  thus  af- 
fected are  said  to  be  polarized. 
what  arc  the  The  name  poles  is  given  in  physics  in  gen- 

poieaofabody? 


enjoy,  or  have  acquired  any  contrary  properties. 

Thus,  the  opposite  ends  or  sides  of  a  magnet  have  contrary  properties,  in- 
asmuch as  each  attracts  what  the  other  repels.  The  opposite  ends  of  an  elec- 
tric or  galvanic  arrangement  are,  for  like  reasons,  denominated  poles.  So  also 
in  the  case  of  light,  the  rays  which  have  been  reflected  or  transmitted  under 


342  WELLS'S  NATURAL  PHILOSOPHY. 

peculiar  conditions  are  said  to  possess  poles,  because  in  some  positions  they 
can  be  reflected  and  hi  others  they  can  not,  and  these  positions  are  at  right 
angles  to  one  another. 

Explain  the  dis-  '*02*  Tne  Phenomenon  of  polarized  light  was  discovered  in 
coveryandphe-  1808,  by  Malus,  a  young  engineer  officer  of  Paris.  Ou  one 
occasion,  as  he  was  viewing  through  a  double  refracting 
prism  of  Iceland  spar  the  light  of  the  sun  reflected  from  a  glass 
window  in  one  of  the  French  palaces,  he  observed  some  very  peculiar  effects. 
The  window  accidentally  stood  open  like  a  door  on  its  hinges  at  an  angle  of 
54°,  and  Malus  noticed  that  the  light  reflected  from  this  angle  was  entirely 
altered  in  its  character. 

This  alteration  in  the  character  of  the  light  reflected  from  the  glass  window, 
which  was  thus  first  observed  by  Malus,  may  be  made  clear  by  the  following 
experiment : — Suppose  we  have  a  cylinder  with  a  mirror  at  one  end  of  it  If 
we  point  this  to  the  sun,  and  receive  the  image  on  a  distant  screen,  we  may 
turn  the  cylinder  round  on  its  axis,  and  the  reflected  ray  will  be  found  to  revolve 
constantly  with  it.  But  if  now,  instead  of  receiving  the  ray  direct  from  the  sun, 
we  allow  a  beam  reflected  from  a  glass  plate,  at  an  angle  of  about  54°,  to  fall 
upon  the  mirror,  and  then  be  reflected  on  the  screen,  it  will  be  found  that  the 
point  of  light  will  not  have  the  same  properties  as  that  previously  examined ; 
it  will  be  altered  in  its  degree  of  intensity  as  the  cylinder  turns  round ;  will 
have  points  where  it  is  very  bright,  and  others  where  it  will  entirely  disap- 
pear. It  is  thus  proved  that  light  reflected  from  glass  at  an  angle  of  about 
64°,  has  undergone  some  peculiar  modification,  or,  as  it  has  been  termed, 
has  become  polarized. 

Certain  minerals,  especially  those  called  "tourmalines,"  have  the  prop- 
erty of  polarizing  a  ray  of  light  transmitted  through  them. 

FiG.  278.  If  a  ray  of  light  be  caused  to  pass  through 

a  thin  plate  of  tourmaline,  as  c  d,  Fig.  278, 
in  the  direction  of  the  line  a  b,  and  be  re- 
ceived upon  a  second  plate,  e  f,  placed 
symmetrically  with  the  first,  it  passes 
through  both  without  difficulty;  but  if  the 
second  plate  be  turned  a  quarter  round,  as 
in  the  direction  g  h,  the  light  is  totally  cut  off. 

According  to  the  undulatory  theory,  the  dif- 
ference between  common  and  polarized  light 
e*?  may  be  explained  by  supposing  that  in  com- 

mon light  the  vibrations  of  the  ether  which  produce  it 
take  place  in  every  possible  direction,  transverse  to  the 
path  of  the  ray ;  but  in  polarized  light  they  take  place 
in  only  one  direction,  or  are  all  in  one  plane. 

Thus,  in  the  passage  of  a  ray  of  light  through  the  plate  of  tourmaline, 
only  one  set  of  vibrations  is  transmitted,  while  the  others  are  absorbed, 


THE   ANALYSIS   OF   LIGHT.  343 

The  transmitted  raj,  having  all  its  vibrations  in  one 
direction,  readily  passes  through  a  second  plate  of 
tourmaline,  the  structural  arrangement  of  which  is 
symmetrical  with  that  of  the  first;  but  if  this  ar- 
rangement be  altered  by  turning  the  plate  partially 
round,  the  vibrations  are  intercepted.  In  the  same 
manner  a  sheet  of  paper,  c  d,  Fig.  279,  may  be  slipped 
through  a  grating,  a  6,  its  plane  coinciding  with  the  length  of  the  bars ;  but 
can  no  longer  go  through  when  it  is.  turned,  as  at  e  f,  a  quarter  round. 

Light  -is  polarized  by  reflection  from  many 

Is  light  polar-        ,.«,  , •     ,  i  -,  . 

ized  by  renec-  different  substa/ices,  such  as  glass,  water,  air, 
substanSs  e  ebony,  mother-of-pearl,  surfaces  of  crystals, 
etc.,  etc.,  provided  that  the  light  falls  at  a 
certain  angle  peculiar  to  each  surface.  This  angle  is 
called  the  polarizing  angle.* 

What  are  some  Sine®  the  discovery  of  polarized  light,  its  principles  have 
of  the  practical  been  applied  to  the  determination  of  many  practical  results. 

ThuS'  [i  haS  been  f°Und  that-  aU  reflected  %ht>  come  from 
whence  it  may,  acquires  certain  properties  which  enable  us 
to  distinguish  it  from  direct  light ;  and  the  astronomer,  in  this  way,  is  en- 
abled to  determine  with  infallible  precision  whether  the  light  he  is  gazing  on 
(and  which  may  have  required  hundreds  of  years  to  pass  from  its  source  to 
the  eye),  is  inherent  in  the  luminous  body  itself,  or  is  derived  from  some  other 
source  by  reflection.  It  has  been  also  ascertained  by  Arago  that  light  pro- 
ceeding from  incandescent  bodies,  as  red-hot  iron,  glass,  and  liquids,  under  a 
certain  angle,  is  polarized  light ;  but  that  light  proceeding,  under  the  same 
circumstances,  from  an  inflamed  gaseous  substance,  such  as  is  used  in  street 
illumination,  is  always  in  a  natural  state,  or  unpolarized.  Applying  these 
principles  to  the  sun,  he  discovered  that  the  light-giving  substance  of  this 
luminary  was  of  the  nature  of  a  gas,  and  not  a  red-hot  solid  or  liquid  body. 

In  a  similar  manner  the  chemist  is  able  to  determine,  by  the  manner  in 
which  light  is  reflected  or  polarized  by  a  crystallized  body,  whether  it  has 
been  adulterated  by  the  addition  of  foreign  substances. 

what  three  V  ?03.  Solar  light;  in  addition  to  the  lumin- 
taSSded"  ™£  /^us  principle  which  produces  the  phenomena  of 
soiar  light?  /  coior  an(j  js  faQ  cause  Of  vision,  contains  two 

other  principles,  viz.,  heat  and  actinism,  or  the  chem- 
ical principle.  These  principles  are  invisible  to  the  eye, 
and  have  only  been  discovered  by  their  effects  on  other 
bodies. 

*  The  phenomena  of  polarized  light  are  so  abstruse,  and  depend  to  so  great  an  extent 
on  experimental  illustration  for  their  proper  comprehension,  that  an  extended  descrip- 
tion of  them  in  an  elementary  work  is  impossible. 


344  WELLS'S   NATURAL   PHILOSOPHY. 

The  constitution  of  the  solar  ray  may  be  compared  to  a  bundle  of  three  sticks, 
one  of  which  represents  heat,  another  light,  and  a  third  the  actinic  principle. 
"We  know  that  these  three  principles  exist  in  every  ray  of 
know-  rtiat  BO-  so^ar  h'ght»  because  we  are  able  to  separate  them  in  a  great 
lar  light  con-  degree  from  each  other.  Thus  the  luminous  principle  passes 
principles?  re  readily  through  a  transparent  plate  of  alum,  but  nearly  all  the 
heat  is  absorbed.  Certain  dark-colored  bodies,  on  the  con- 
trary, allow  nearly  all  the  heat  to  pass,  but  obstruct  the  light.  A  blue  glass 
obstructs  nearly  all  the  light  and  heat  of  the  solar  ray,  but  allows  the  chem- 
ical principle  to  pass  freely;  while  a  yellow  glass  allows  light  and  heat  to 
pass,  but  obstructs  the  passage  of  the  chemical  influence. 

When  we  decompose  a  ray  of  solar  light  by 

How    are   the  „  .         *  J  J 

three    princi-    means  of  a  prism,  and  throw  the  spectrum 

pies     of    solar  .        ,  .  ,       .„  - 

light   affected    upon  a  screen,  the  luminous,  the  calormc,  and 
the  actinic  radiations  will  each  assume  a  dif- 
ferent position.     All  will  be  refracted  by  passing  through 
the  prism,  but  in  different  degrees. 

The  calorific,  or  heat  radiations  will  be  refracted  least,  and  their  maximum 
point  will  be  found  but  slightly  thrown  out  of  the  right  line  which  the  solar 
ray  would  have  traversed  had  it  not  been  intercepted  by  the  prism.  The 
beat  diminishes  with  much  regularity  on  each  side  of  this  line. 

The  luminous  radiations  are  subject  to  a  greater  degree  of  refraction  ;  their 
point  of  maximum  intensity  being  in  the  yellow  ray,  lying  considerably  above 
the  point  of  greatest  heat.  The  light  diminishes  on  each  side  of  it,  producing 
orange,  red,  and  crimson  colors  below  the  maximum  point,  and  green,  blue, 
and  violet  above  it. 

The  radiations  which  produce  chemical  action  are  more  refrangible  than 
either  the  calorific  or  luminous  radiations,  and  the  maximum  of  chemical 
power  is  found  at  that  point  of  the  spectrum  where  light  is  feeble,  and  where 
scarcely  any  heat  can  be  detected. 

The  positions  in  the  spectrum  of  the  heat  and  actinic  radiations,  which  are 
invisible  to  the  eye,  may  be  found  by  experiment.  Thus,  if  we  place  a  deli- 
cate thermometer  in  the  different  rays  of  the  spectrum  (§  686,  Fig.  268),  it 
will  be  found  that  the  indigo  and  violet  rays  scarcely  affect  it  all,  while  the 
yellow  ray,  which  is  the  most  luminous,  is  inferior  in  heating  action  to  the 
red  ray,  which,  yielding  but  little  light,  possesses  the  greatest  amount  of  heat. 
If  now,  the  thermometer  be  carried  a  little  below  and  just  out  of  the  red 
ray,  into  the  darkened  space,  it  will  exhibit  the  greatest  increase  in  tempera- 
ture, thus  proving  the  presence  of  a  heating  ray  in  solar  light,  independent 
©f  the  luminous  ray.  In  a  like  manner,  by  substituting  a  chemically  prepared 
surface,  as  a  piece  of  photographic  paper,  for  the  thermometer,  the  presence 
of  a  chemical  ray  can  be  proved  in  the  darkened  space  at  the-  other  end  of  the 
gpectrum,  and  near  to  the  blue  and  violet  rays. 

704.  Those  rays  of  solar  light  which  are  less  refrangible  than  any  of  the 


THE  ANALYSIS  OF  LIGHT.  345 

visible  colored  rays  of  the  spectrum,  have  all  the  properties  of  radiant  heat 
coming  from  bodies  of  a  lower  temperature  than  800°  F.  Such  heat  is  much 
less  refrangible  than  red  light ;  but  if  the  temperature  of  the  radiating  body 
be  increased,  it  emits,  in  addition  to  the  rays  previously  emitted,  others  of  a 
higher  refrangibility,  until  at  last  some  few  of  its  rays  become  as  refrangible 
as  the  least  refrangible  rays  of  light.  The  body  then  appears  of  the  same 
color  as  the  least  refrangible  rays  of  light,  and  is  said  to  be  red  hot.  If  it 
be  heated  more,  it  emits,  in  addition  to  the  red,  still  more  refrangible  rays, 
viz.,  orange ;  then  (at  a  higher  temperature)  yellow  rays  are  added,  and  so 
on,  until  when  the  body  is  white  hot,  it  emits  all  the  colors  visible  to  us  ; 
and  in  some  instances  (of  very  intense  heat),  even  the  invisible  chemical  rays, 
more  refrangible  than  the  violet,  are  emitted,  though  in  less  quantity  than 
in  the  solar  rays.  Thus  light  appears  to  be  nothing  more  than  visible  heat, 
and  heat  invisible  light — the  constitution  of  the  eye  being  such  that  it  can 
perceive  one  and  not  the  other,  in  the  same  way  as  the  ear  can  appreciate 
vibrations  of  sound  more  rapid  than  sixteen  per  second,  but  not  those  which 
are  less  rapid. 

What  curious  *705'  The  stud7  of  tne  chemical  principle  contained  in  the 
fact  has  the  rays  of  solar  light  has  rendered  probable  the  curious  fact,  that 
ch*  nncal^  rin6  no  su^stance  can  be  exposed  to  the  sun's  rays  without  un- 
cipie  of  light  dergoing  a  chemical  change  ;  and  from  numerous  examples  it 
evolved  ?  would  seem  that  the  changes  in  the  molecular  condition  of 

bodies  which  sunlight  effects  during  the  daytime,  is  made  up  during  the 
hours  of  night,  when  the  action  is  no  longer  influencing  them.  Thus  dark- 
ness appears  to  be  essential  to  the  healthy  condition  of  all  organized  and  un- 
organized forms  of  matter. 

upon  what  does  The  process  of  forming  Daguerreotype  and 
ofphStograpMc  other  photographic  pictures,  depends  solely 
pictures  depend?  u^on  the  actinic?  or  cnemical  influence  of  the 

solar  ray. 

The  term  "photography,"  signifying  light  drawing,  which  is  the  general 
name  given  to  this  art,  is  unfortunate  and  ill-chosen,  for  not  only  does  light 
not  exercise  any  influence  in  producing  the  pictures,  but  it  tends  to  destroy 
them. 

What  are  the  The  essential  steP3  of  tne  process  of  forming  a  Daguerre- 
essential  steps  otype  picture  consist  in  coating  a  suitable  plate  of  metal  with 
°uerrteol  e^"  some  chemical  compound  easily  affected  by  the  action  of  the 
process  ?  solar  ray.  Such  a  coating  is  usually  a  compound  of  the  ele- 

mentary body  Iodine.  The  plate  is  then  exposed  to  the  imago 
formed  by  the  lens  of  a  camera  obscura.  Relatively,  the  quantity  of  light  and 
actinism  reflected  from  any  object  are  the  same ;  therefore  as  the  light  and 
shadows  of  the  luminous  image  vary,  so  will  the  power  of  producing  change 
upon  the  plate  vary,  and  the  result  will  be  the  production  of  an  image  which 
will  be  a  faithful  copy  of  nature,  with  reversed  lights  and  shadows;  the 
lights  darkening  the  plate,  while  the  shadows  preserve  it  white,  or  unaltered. 

15* 


346  WELLS'S  NATURAL  PHILOSOPHY. 

If  the  plate  were  then  left  without  further  care,  the  image  formed  would 
soon  fade  away,  and  leave  no  trace  on  its  surface.  In  practice,  the  plate  is 
not  exposed  to  the  influence  of  light  sufficiently  long  to  form  upon  its  sur- 
face an  image  visible  to  the  eye,  but  the  picture  is  developed,  or  brought  out 
and  rendered  permanent  by  exposure  to  the  vapor  of  mercury.  This  metal, 
in  a  state  of  very  fine  division,  is  condensed  upon  and  adheres  to  those  por- 
tions of  the  surface  of  the  plate  which  have  been  subjected  .to  the  influence 
of  the  chemical  action.  "Where  the  shadows  are  deep,  there  is  scarcely  a 
trace  of  mercury  ;  but  where  the  lights  are  strong,  the  metallic  dust  is  de- 
posited of  considerable  thickness.  This  deposition  of  mercury  essentially  com- 
pletes and  fixes  the  picture. 

The  reason  why  the  vapor  of  mercury  attaches  itself  only  to  those  portions 
of  the  plate  which  have  been  affected  by  the  chemical  influence  of  light  is  not 
definitely  known:  in  all  probability,  we  have  involved  the  action  of  several 
forces.  It  is  not,  however,  necessary  that  a  surface  should  be  chemically  pre- 
pared to  exhibit  these  results.  A  polished  plate  of  metal,  a  piece  of  marble, 
of  glass,  or  even  wood,  when  partially  exposed  to  the  action  of  light,  will, 
when  breathed  upon,  or  presented  to  the  action  of  mercurial  vapor,  show  that 
a  disturbance  has  been  produced  upon  the  portions  which  were  illuminated  ; 
whereas  no  change  can  be  detected  upon  the  parts  kept  in  the  dark. 

That  the  luminous  principle  is  not  necessary  for  the  success 
*  °^  ^e  Pn°t°graphic  process,  may  be  proved  by  the  experi- 


ment 

that   light   is     ment  of  taking  a  daguerreotype  in  absolute  darkness.     This 

fo^thTproE  can  be  accomplished  in  the  following  manner  :  —  A  large  pris- 
tion  of  a  pho-  matic  spectrum  is  thrown  upon  a  lens  fitted  into  one  side  of  a 
suft?P  "  dark  chamber;  and  as  the  actinic  power  resides  in  great  ac- 

tivity at  a  point  beyond  the  violet  ray,  where  there  is  no  light, 
the  only  rays  allowed  to  pass  the  lens  into  the  chamber  are  those  beyond  the 
limit  of  coloration,  and  non-luminous  ;  these  are  directed  upon  any  object,  and 
from  that  object  radiated  upon  a  highly  sensitive  photographic  surface.  In 
this  way  a  picture  may  be  formed  by  radiations  which  produce  no  effect  upon 
the  eye. 

.^       _  706.  There  are  many  reasons  for  supposing  that  each  of  the 

do  the  three  three  principles,  light,  heat,  and  actinism,  included  in  the  solar 
thenC6olar  ra^  rs^>  exerc^se  a  distinct  and  peculiar  influence  upon  vegeta- 
exert  on  vege-  tion.  Thus  the  luminous  principle  controls  the  growth  and 

coloration  of  plants,  the  calorific  principle  their  ripening  and 
fructification,  and  the  chemical  principle  the  germination  of  seeds.  Seeds 
which  ordinarily  require  ten  or  twelve  days  for  germination,  will  germinate 
under  a  blue  glass  in  two  or  three.  The  reason  of  this  is,  that  the  blue  glass 
permits  the  chemical  principle  of  light  to  pass  freely,  but  excludes,  in  a  great 
measure,  the  heat  and  the  light.  On  the  contrary,  it  is  nearly  impossible  to 
make  seeds  germinate  under  a  yellow  glass,  because  it  excludes  nearly  all 
the  chemical  influence  of  the  solar  ray. 


THE   EYE,    AND   THE  PHENOMENA   OF    VISION.        347 


SECTION   IV. 


THE     BYE.     AND     THE     PHENOMENA     OP    VISION. 


If  an  openi 
be  made  in  thi 
side  of  a  dart 
chamber  how 
will  images  of 
external  ob- 
jects be  repre- 
sented ? 


TOt.  If  -we  make  a  small  aperture  through  the  shutter  of  a 
darkened  room,  the  images  of  external  objects  will  be  pic- 
tured indistinctly,  and  in  an  inverted  position,  upon  the  op- 
posite walL  The  reason  of  this  will  appear  evident  from  an 
inspection  of  Fig.  280.  It  will  be  seen  that  the  rays  of  light 
diverging  from  the  top  and  bottom  of  the  object  cross  each 
other  in  passing  through  the  aperture,  and  consequently  form  an  inverted 
image.  This  image  is  rendered  more  distinct  with  a  small  aperture  than  with 
a  large  one,  since,  in  the  first  case,  the  rays  which  proceed  from  any  particu- 
lar part  of  the  object  fall  only  upon  the  corresponding  part  of  the  image,  and 
are  not  scattered  indiscriminately  over  the  whole  picture,  as  they  would  be 
if  the  aperture  was  larger. 

FiG.  280. 


Describe  the  "^  *n  *ne  P^ace  °^  *ne  room  with  an  aperture  in  the  shutter, 
construction  of  we  substitute  a  dark  box,  with  a  double-convex  lens  fitted 
Obscura.amera  into  one  side>  a  picture  will  be  formed  on  the  opposite  side  of 
the  box,  or  upon  a  screen  placed  at  the  focal  distance  of  tho 
lens.  This  picture  will  represent,  with  great  beauty  and  distinctness,'  whatever 
is  in  front  of  the  lens,  all  the  objects  having  their  proper  relations  of  light  and 
shadow,  and  their  proper  colors.  Such  an  apparatus  is  called  a  CAMERA 
OBSCURA. 

Fig.  281  represents  the  ordinary  construction  of  the  camera  obscura.  It 
consists  of  a  wooden  rectangular  box,  into  which  the  rays  of  the  light  penetrate 
through  a  convex  lens  placed  at  the  termination  of  the  tube  B.  These  rays, 
if  unobstructed,  will  form  an  image  upon  the  opposite  side  of  the.  box  0,  but 
if  they  are  received  upon  a  mirror,  M,  inclined  at  an  angle  of  45°,  their  direc- 
tion is  changed,  and  the  image  will  be  formed  upon  a  screen,  or  plate  of 
ground  glass,  N,  placed  at  the  top  of  the  box.  By  placing  upon  this  screen  a 
sheet  of  tracing  paper,  the  outlines  of  the  image  may  be  readily  copied. 


348 


WELLS'S   NATURAL   PHILOSOPHY. 


Such  a  modification  of  the  camera  is  very  convenient  for  artists  and  traveler! 
in  sketching  landscapes,  etc. 

FiG.  281. 


How  does  the 
eye  resemble 
the  camera 
obscura ? 


What    is    the 


708.  The  mechanical  arrangement  of  the 
eye  in  man  and  the  higher  animals  is  the  same 
as  that  of  the  camera  obscura,  being  simply  a 
double-convex  lens,  fitted  into  one  side  of  a  spherical 
chamber,  through  which  the  rays  of  light  pass  to  form  an 
inverted  picture  upon  the  back  of  the  chamber.*' 

^n  man?  the  organs  of  vision  consist  of  two 
hereye  h°U°w  spheres,  each  about  an  inch  in  diam- 
eter?  fi]}eci  wjth  certain  transparent  liquids,  and 
deposited  in  cavities  of  suitable  magnitude  and  form,  in 
the  upper  part  of  the  front  of  the  head  on  each  side  of 
the  nose. 

The  sphere  of  the  eye,  or  the  eye-ball,  is 
moved  in  its  socket  by  muscles  attached  to 
different  points  of  its  surface,  so  that  it  is 
capable  of  being  moved  within  certain  limits 
in  every  direction. 

*  This  may  be  proved  by  taking  the  eye  of  a  recently-killed  bullock  and  cutting  a  small 
hole  in  the  upper  part  of  the  ball,  looking  into  the  interior. 


In  niiiu  ? 


How  are  •we 
enabled  to 
move  the  eye 
in  different  di- 
rections? 


THE  EYE,   AND   THE  PHENOMENA   OF  VISION. 


349 


FiG.  282.  The  arrangement  of  theso 

muscles  is  shown  in  Fig.  282, 
where  the  external  bones  of 
the  temple  are  supposed  to  be 
removed  in  order  to  render 
them  visible.  The  muscle,  1, 
raises  the  eyelid,  and  is  con- 
stantly in  action  while  we  are 
awake.  During  sleep,  the 
muscle  being  in  repose  and 
relaxed,  the  eye-lid  falls  and 
protects  the  eye  from  the  ac- 
10  tion  of  light.  The  muscle,  4, 
turns  the  eye  upward;  5, 
downward;  6,  outward;  and 
a  corresponding  one  on  the  in- 
side, not  seen  in  the  figure, 
turns  it  inward.  Nos.  2  and 
10  turn  the  eye  round  its  axis. 

ef  what  parts  * —  eye  consists  essentially  of  four  coats,  or 
consist?6  eye  membranes,  called  the  SCLEROTIC  coat,  the 
CHOROID  coat,  the  CORNEA,  and  the  KETINA  ; 
and  these  coats  inclose  three  transparent  liquids,  called  hu- 
mors— the  AQUEOUS  humor,  the  VITREOUS  humor,  and  the 
CRYSTALLINE  humor,  the  last  of  which  has  the  form  of  a  lens. 
The  Sclerotic  coat  is  the  external  coat  of  the 


Describe   the 

Sclerotic  coat. 

of  the  form  of  the  eye  chiefly  depends. 

It  is  a  strong,  tough 
membrane,  and  to  it  the 
muscles  which  move  the 
eye  are  attached.  It  cov- 
ers about  four  fifths  of  the 
external  surface  of  the 
eye-ball,  leaving,  however, 
two  circular  openings,  one 
before  and  the  other  be- 
hind the  eye.  Its  position 
is  shown  at  t,  Fig.  283. 

What    is    the 
Cornea? 


tae 


is  the  clear,  trans- 
parent   coat  which 


350  WELLS'S  NATURAL  PHILOSOPHY. 

forms  the  front  of  the  eye-ball.  It  is  firmly  united  to,  or 
fixed  in  the  sclerotic  coat,  like  the  glass  in  the  case  of  a 
watch. 

The  Cornea  is  represented  at  a,  Pig.  283. 

what  is  the         The  Choroid  coat  is  a  delicate  membrane, 

Choroid  Coat?       J—^    ^    ^^  gurface  Qf   the    gclerotic  CQat? 

and  covered  on  the  interior  with  a  black  pigment. 

It  is  represented  at  k,  Fig.  283. 

what  is  the        The  Eetina  is  a  delicate,  transparent  mem- 
Betina?        fcrane  which  spreads  over  the  chief  part  of  the 
internal  surface  of  the  eye-ball,  and  is  situated  imme- 
diately within  and  close  to  the  choroid  coat. 

The  position  of  the  Retina  is  shown  at  m,  Fig.  283. 

HOW  is  the  re-  The  retina  is  formed  by  the  expansion  of  a 
tina  formed?  nerve  called  the  optic  nerve,  which  proceeds 
from  the  back  of  the  eye  through  the  bones  of  the  skull 
into  the  brain,  and  conveys  to  the  brain  the  impressions 
made  by  external  objects  on  the  organs  of  vision.  If  this 
nerve  were  divided,  notwithstanding  the  eye  might  be  in 
other  respects  perfect,  the  sense  of  sight  would  be  de- 
stroyed. 

No.  11,  Fig.  282,  and  n,  Fig.  283,  exhibit  the  relative  position  of  the 
optic  nerve. 

what  is  the  ^n  looking  into  the  eye  from  without,  we 
iris?  perceive  a  flat,  circular  membrane,  which,  in 
different  eyes,  is  of  a  black,  blue,  or  gray  color.  This 
membrane  is  called  the  IRIS,  and  divides  the  eye  into  two 
very  unequal  portions. 

The  Iria  is  represented  at  c  d,  Fig.  283. 

The  Pupil  of  the  eye  is  the  circular  black 

What    is    the  .  .    F._  J  .    .  . 

Pupil  of  the    opening  in  the  center  of  the  ins,  and  is  the 
space  through  which  light  is  admitted  into 
the  interior  of  the  eye. 

The  open  space  between  c  and  d,  Fig.  283,  represents  the  pupil.  It  is, 
properly  speaking,  the  window  of  the  eye,  and  appears  black,  only  because 
the  chamber  within  and  behind  it  is  dark.  When  a  small  quantity  of  light 
enters  the  eye  the  pupil  widens  or  expands;  but  when  a  large  quantity  enters, 
it  closes  or  contracts. 


THE  EYE,   AND  THE  PHENOMENA   OF   VISION.        351 

The  two  parts  into  which  the  iris  divides  the  eye  are 
called  the  anterior  and  posterior  chambers. 
what  are  the        The  anterior  chamber,  or  the  space  before 
SreoSI    hu*    tae  iri^  is  ^e(^  witn  a  fl11^  resembling  pure 
more?  water,  and  therefore  called  the  aqueous  hu- 

mor ;  and  the  posterior  chamber,  or  the  space  behind  the 
iris,  is  filled  with  a  thick  liquid,  somewhat  resembling  the 
white  of  an  egg,  called  the  vitreous  humor. 

In  Fig.  283,  b  e  represents  the  aqueous  humor,  and  h  the  vitreous  humor, 
this  last  occupying  all  the  interior  of  the  chamber  of  the  eye. 

The  crystalline  lens  is  composed  of  a  more  solid  sub- 
stance than  either  the  aqueous  or  vitreous  humor.  It  is 
inclosed  within  a  transparent  bag,  or  capsule,  having  the 
form  of  a  double-convex  lens,  and  is  suspended  imme- 
diately behind  the  iris,  and  between  the  aqueous  and 
vitreous  humors. 

Its  form  and  position  are  represented  at/  Fig.  283. 

HOW  do  we  by  ^Q9.  Rays  of  light  proceeding  from  an  ob- 
Se  eygeanpe?-f  Ject  and  entering  the  eye,  are  refracted  by  the 
ceive objects?  C0rnea  and  crystalline  lens,  and  made  to  con- 
verge to  a  focus  at  the  back  of  the  eye,  and  form  an 
image  upon  the  retina.  This  image,  by  producing  a  sen- 
sation upon  the  optic  nerve,  conveys  in  some  unknown 
way  to  the  mind  a  perception  and  knowledge  of  the  ex- 
ternal object. 

Fig.  284  represents  the  manner  in  FiQ.  284. 

which  the  image  is  formed  upon  the 
retina  in  the  perfect  eye.  The  curva- 
ture of  the  cornea,  s  s,  and  of  the 
crystalline  lens,  c  c,  is  just  sufficient 
to  cause  the  rays  of  light  proceeding 
from  the  image,  1 1',  to  converge  to 
the  right  focus,  m  m,  upon  the  retina. 

when  does  di«-  Distinct  vision  can  only  take  place  in  the 
piacV?isi(mtake  eve  wnen  tne  cornea  and  crystalline  lens  have 
such  convexities  as  to  bring  the  rays  of  light 
proceeding  from  an  object  to  an  exact  focus  upon  the 
retina. 


352 


WELLS'S   NATURAL  PHILOSOPHY. 


How  is  the  eye 
enabled  to  see 

Sncttyat  differ'-' 
ent  distances  ? 


what  is  the 


FIG.  285. 


As  the  rays  of  light  proceeding  from  distant  objects  enter 
the  eye  at  different  angles,  they  will  naturally  tend  to  meet 
at  different  foci  after  refraction  by  the  crystalline  lens,  and 
thus  form  indistinct  images.  This  is  remedied  by  a  power 
which  the  eye  possesses  of  adapting  itself  to  the  direction  of 
the  light  proceeding  from  various  distances,  so  that  in  the  healthy  eye,  rays 
coming  from  near  and  distant  objects  are  all  equally  converged  to  a  focus  on 
the  same  point  of  the  retina.  How  the  eye  effects  this  is  not  certainly  known, 
but  it  is  supposed  to  be  by  increasing  or  diminishing  the  sphericity  of  the 
crystalline  lens  and  cornea. 

A  person  is  said  to  be  near-sighted  when 
the  curvature  of  the  cornea  and  crystalline 
lens  is  so  great,  that  the  rays  of  light  which 
form  the  image  are  brought  to  a  focus  before  they  reach 
the  retina,  or  the  back  part  of  the  eye.  The  object,  there- 
fore, is  not  distinctly  seen. 

Fig.  285  represents  the  manner 

in  which  the  image  is  formed  in 

the  eye  of  a  near-sighted  person. 

The  curvature  of  the  cornea,  s  5, 

and  of  the  crystalline  lens,  c  c,  is 

so  great  that  the  image  is  formed 

at  m  m,  in  advance  of  the  re- 
tina. 

Short-sightedness  is  remedied  either  by  holding  the  object 
nearer  to  the  eye,  or  by  the  employment  of  spectacles  tho 
glasses  of  which  are  concave  lenses.  In  both  cases  the  rays 

proceeding  from  the  object  enter  the  eye  with  a  greater  degree  of  divergence, 

and  therefore  do  not  converge  so  soon  to  a  focus. 

A  person  is  said  to  be  far-sighted  when,  on 

What    ia    the  e  ' 

e»u«e  of  far-    account  of  a  flattening  of  the  cornea  and  the 
crystalline  lens,  the  rays  of  light  do  not  con- 
verge sufficiently  to  form  a  distinct  image  upon  the  retina. 

ogg  Fig.  286,  represents  the  manner 

in  which  the  image  is  formed  in 
the  eye,  when  the  cornea  or  crys- 
talline lens  is  flattened.  The  per- 
fect image  would  be  produced  at 
77i  wi,  behind  the  retina,  and,  of 
course,  beyond  the  point  necessary 
to  secure  distinct  vision. 
How  may  long-  Long-sightedness  may  be  remedied  by  the  employment  of 
remedied?6  **  spectacles,  the  glasses  of  which  are  convex  lenses.  These,  by 


How  is  Bhort 
sightednesg0 
remedied  ? 


THE   EYE,    AND   THE   PHENOMENA   OF    VISION.        353 

increasing  the  convergence  of  rays  of  light  passing  through  them,  bring  them 
sooner  to  a  focus  in  the  eye,  and  thus  produce  the  image  upon  the  right  point 
of  the  retina.* 

Most  persons  of  advanced  age  are  troubled  with  long-sightedness,  and  are 
obliged  to  use  spectacles.  The  reason  of  this  is,  that  as  the  physical  organi- 
zation of  the  body  becomes  enfeebled,  the  humors  of  the  eye  dry  up,  or 
are  absorbed,  and  in  consequence  of  this,  the  cornea  and  crystalline  lens 
shrink  and  become  flattened. 

Beside  these  defects  of  the  eye,  a  person  may  have  the  sense  of  vision 
impaired  or  destroyed  by  an  injury  or  disease  of  the  optic  nerve,  or  by  a  dimi- 
nution of  the  transparency  of  the  crystalline  lens ;  the  first  of  these  cases  is 
called  amaurosis,  and  is  incurable — the  second,  which  is  called  cataract,  may 
be  cured. 

As  the  images  The  imaoes  formed  by  the  rays  of  light  upon  the  retina  are 
on  the  retina  inverted.  It  may,  therefore,  bo  asked  why  all  visible  objects 
why  do^we^not  ^°  no*  aPPear  upside  down?  The  explanation  of  this  curious 
see  them  up-  point,  which  has  formed  the  subject  of  much  dispute,  appears 
to  be  this:  an  object  appears  to  be  inverted  only  as  it  is  com- 
pared with  some  other  objects  which  are  erect.  If  all  objects  hold  the  same 
relatiye  position,  none  can  be  properly  said  to  be  inverted.  Now,  since  all 
the  images  produced  upon  the  retina  hold,  with  relation  to  each  other,  tho 
same  position,  none  are  inverted  with  respect  to  others ;  and  as  such  images 
alone  can  be  the  object  of  vision,  no  one  object  of  vision  can  be  inverted  with 
respect  to  any  other  object  of  vision ;  and,  consequently,  all  being  seen  in  the 
same  position,  that  position  is  called  the  erect  position. 

710.  The  optic  axis  of  the  eye  is  a  line 

What   is    the        ,  \.  J. 

optic  axis  of    drawn  perpendicularly  through  the  center  of 
the  cornea,  and  center  of  the  eye-ball. 

^he  reason  why  with  two  eyes  we  do  not  see 
double  is,  because  the  axis  of  both  eyes  is 
ject double?  turned  to  one  point,  and  therefore  the  same 
impression  is  made  on  the  retina  of  each  eye. 

The  law  of  vision  for  visible  objects  is  entirely  different  from  that  for  points. 
A  visible  object  can  not,  in  all  its  parts,  be  seen  single  at  the  same  instant  of 
tune,  but  the  two  eyes  converge  their  axes  to  the  near  and  the  remote  parts  of 
it  in  succession,  and  thus  give  an  idea  of  the  different  distances  of  its  parts. 
Any  defect  which  will  prevent  the  two  eyes  from  moving  together  conjointly, 
and  from  converging  their  optic  axes  upon  every  point  of  an  object  in  succes- 
sion, will  be  fatal  to  distinct  vision. 

•  Birds  of  prey  are  enabled  to  adjust  their  eyes  so  as  to  see  objects  at  a  groat  distance, 
and  again  those  which  are  very  near.  The  first  is  accomplished  by  means  of  a  muscle  in 
the  eye,  which  permits  them  to  flatten  the  cornea  by  drawing  back  the  crystalline  lens ; 
and  to  enable  them  to  perceive  distinctly  very  near  objects,  their  eyes  are  furnished  with 
a  flexible  bony  rim,  by  which  the  cornea  is  thrown  forward  at  will,  and  the  eye  thus  ren- 
dered near-sighted. 


354  WELLS'S  NATURAL  PHILOSOPHY. 

HOW  ma  dou  Double  vision  may  be  produced  by  pressing 
be  ^igk^y  fr°m  the  ^de  uPon  ^e  ball  of  either 
eye  while  viewing  an  object ;  the  pressure  of 
the  finger  prevents  the  ball  of  one  eye  from  following  the 
motion  of  the  other,  and  the  axis  of  vision  in  each  eye 
being  rendered  different,  we  see  two  images. 

Strabismus,  or  squinting,  is  caused  by  the  inability  of  one  eye  to  follow  the 
motions  of  the  other,  and  persons  so  affected  always  see  double ;  practice, 
however,  gives  them  power  of  attending  to  the  sensation  of  only  one  eye  at  a 
time. 

It  is  from  this  inability  of  the  eye  to  fix  its  optical  axis  that  drunkards  see 
double. 

HOW  do  we         711>  We  .iudge  of  the  distaiice  and  size  of 
judge  of  the    an  object  by  the  relative  direction   of  lines 

distance      and  "  . 

size  of  an  ob-    drawn  from  the  object  to  the  eye,  and  by  the 
angle  which   the  intersection   of  these   lines 
makes  with  the  eye.     This  angle  is  called  the  angle  of 
vision. 

FIG.  287. 
A 


The  student  will  bear  in  mind  that  an  angle  is  simply  the 

anguTof  vision?     inclination  of  two  lines  without  any  regard  to  their  length. 

Thus,  in  Fig.  287,  the  lines  drawn  from  A  and  B,  C  and  D, 

which  may  be  supposed  to  represent  rays  of  light,  meet  at  the  eye,  and  form 

an  angle  at  the  point  of  intersection.     This  angle  is  the  angle  of  vision. 

If  A  B,  Fig.  287,  represent  a  man  on  a  distant  mountain,  or  on  a  church 
steeple,  and  C  D  a  crow  close  by,  the  angle  formed  by  the  inclination  of  the 
lines  proceeding  from  the  two  objects  will  be  equal,  or  the  line  A  B,  which  is 
the  height  of  the  man,  will  subtend  the  same  angle  as  the  line  C  D,  which  is 
the  height  of  the  crow ;  and  therefore  the  man  appears  at  such  a  distance  no 
larger  than  a  crow. 

How    is    the         The  nearer  an  obJect  is  to  tne  eve>  tne  greater  must  be  the 
angle  of  vision      inclination  of  the  lines  drawn  from  its  extremities  to  intersect 

distance?     b7      and  form  an  angle  at  the  eve'  and  conse(luently  tne  greater 
will  be  its  angle  of  vision.     On  the  contrary,  the  more  remote 


THE   EYE,   AND   THE   PHENOMENA   OF   VISION.        355 


an  object  is  from  the  eye,  the  less  will  be  the  inclination  of  the  lines,  and  the 
less  the  angle  of  vision.  The  nearer  an  object  is  to  the  eye,  therefore,  the 
larger  it  will  appear. 

FIG.  288. 


Thus  the  trees  and  houses  far  down  a  street  or  avenue  appear  smaller  than 
those  near  by,  and  the  size  of  a  vessel  seen  at  sea  diminishes  with  the  increase 
of  distance,  as  is  shown  in  Fig.  288.  The  moon,  on  account  of  its  proximity, 
appears  much  larger  than  any  of  the  stars  or  planets,  although  it  is,  in  fact, 
very  much  smaller. 

FiG.  289. 


when  win  an 


Let  A  B,  Fig.  289,  represent  a  planet,  and  C  D  the  moon.  The  angle  of 
vision  which  the  planet  A  B  makes  with  the  eye  at  Gr,  is  evidently  less  than 
the  angle  which  the  moon  subtends  at  the  same  point.  To  a  spectator  at  Gr, 
therefore,  A  B,  though  much  the  larger  body,  will  appear  no  larger  than 
E  F  ;  whereas  the  moon,  C  D,  will  appear  as  large  as  the  line  C  D. 

?12.  When  an  abject   is  so  remote,  or  so 
small,  that  lines  drawn  from  its  extremities 
form  no  appreciable  angle  at  the  eye,  the  ob- 
ject appears  as  a  mere  speck  or  point. 

^ne  eJe>  w^  an  ordinary  amount  of  light, 
can  see  an  °bject  which  occupies  in  the  field 
of  view  a  space  of  only  the  sixtieth  of  a  de- 
gree (or  one  minute). 

This  space  is  about  the  100th  of  an  inch  in  a  circle  of  twelve  inches  diameter, 
the  eye  being  supposed  to  be  in  the  center  of  the  circle.  Now  a  body  smaller 
than  this  at  six  inches  from  the  eye,  or  any  thing,  however  large,  placed  so 
fer  from  the  eye  as  to  occupy  in  the  field  of  view  less  space  than  this,  is  in  vis- 


point? 


HO 


356  WELLS'S  NATURAL  PHILOSOPHY. 

ible  to  ordinary  sight.     At  four  miles  off,  a  man  becomes  thus  invisible,  and 
a  pin-head  near  by  will  hide  a  house  on  a  distant  hill.* 

what  do  we  713.  When  we  say  we  see  an  object,  we 
^w^ee  In  mean  that  the  mind  is  taking  cognizance  of  a 
picture  or  image  of  the  object  formed  on  the 
retina.  The  manner  in  which  the  sensation  is  conveyed 
by  the  optic  nerve  to  the  brain,  and  a  knowledge  of  the 
external  object  imparted  to.  the  mind,  is  entirely  un- 
known. 

Does  the  sense  "^s  ^e  P*c*;ure>  or  ima»e  °n  the  retina,  is  formed  on  a  com- 
of  sight  giro  paratively  flat  surface,  the  sense  of  sight  can  not  of  itself  af- 
cepSoi^ofYora  for(i  an^  immediate  perception  of  the  distance,  size,  or  position 
size,  position,  of  external  objects.  This  knowledge  we  gain  by  experience 
derived  from  continued  observation,  and  from  the  other  senses. 
A  young  child  has  no  conception  of  distance,  and  grasps  at  the  moon  as  if 
it  were  an  object  immediately  within  its  reach.  Persons  born  blind  and  re- 
stored to  sight  by  surgical  operations,  although  able  to  see  distinctly,  can  not 
properly  comprehend  any  object  or  prospect  before  them.  "  I  see  men  as 
trees  walking,"  said  the  man  born  blind  when  restored  to  sight.  Individuals 
thus  situated  acquire  the  correct  sense  of  vision  only  by  degrees,  like  infants , 
and  it  is  by  experience  that  they  learn  to  walk  about  among  the  objects 
around  them,  without  the  continual  apprehension  of  striking  themselves 
against  every  thing  they  behold. 

what  is  p«r-        Perspective  is  the  name  given  to  that  science 
spective?        -which  teaches  how  to  draw  on  a  plane  surface 
true  pictures  of  objects  as  they  appear  to  the  eye  from  any 
distance  and  in  any  position. 

The  skill  of  the  artist  consists  in  rightly  applying  the  laws  and  principles 
of  perspective  ;  and  a  picture  is  perfect  to  the  extent  in  which  it  agrees  with 
our  experience  of  the  objects  it  represents. 

714.  Many  optical  and  mental  delusions  are  occasioned 
in  estimating  the  size,  figure,  and  position  of  objects,  by 

•  "  The  smallest  particle  of  a  white  substance  distinguishable  by  the  naked  eye  upon  a 
black  ground,  or  of  a  black  substance  upon  a  white  ground,  is  about  the  l-400th  of  an 
inch  square.  It  is  possible,  by  the  closest  attention,  and  by  the  most  favorable  direction 
of  light,  to  recognize  particles  that  are  only  1 -540th  of  an  inch  square,  but  without  any 
sharpness  or  certainty.  But  particles  which  strongly  reflect  light  may  be  seen  when  not 
half  the  size  of  the  least  of  the  foregoing :  thus,  gold  dust  of  the  fineness  of  1  -1125th  of  an 
inch  may  be  discerned  by  the  naked  eye  in  common  daylight.  When  particles  that  can 
not  be  distinguished  by  themselves  with  the  naked  eye  are  placed  in  a  row,  they  become 
visible ;  and  hence  the  delicacy  of  vision  is  greater  for  lines  than  for  single  particles. 
Thus,  opaque  threads  of  no  more  than  1  -4000th  of  an  inch  across,  or  about  half  the  diam- 
eter of  the  silkworm's  fiber,  may  be  discerned  with  the  naked  eye  when  they  are  held 
toward  the  light"— Dr.  Carpenter. 


THE   EYE,    AND   THE   PHENOMENA   OF    VISION.        357 

an  erroneous  application.  of  the  experience  which  in  ordi- 
nary cases  supplies  true  and  accurate  conclusions. 

Thus,  to  most  persons  a  conflagration  at  night,  however 
the     distant,  appears  as  if  very  near.     The  explanation  of  this  mis- 


distance  of  ^a  take  jg  as  follows  :  —  Light  radiating  from  a  center  rapidly 
night  ?  weakens  as  the  distance  from  the  center  increases,  being,  for 

instance,  only  one  fourth  part  as  intense  at  double  the  dis- 
tance. The  eye  learns  to  make  these  allowances,  and  by  the  clearness  and 
intensity  of  the  light  proceeding  from  the  object,  judges  with  considerable  ac- 
curacy of  the  comparative  distance.  But  a  fire  at  night  appears  uncommonly 
brilliant,  and  therefore  seems  near. 

The  evening-star  rising  over  a  hill-top,  appears  as  if  situated  directly  over 
the  top  of  the  eminence.  The  reason  of  this  also  is,  that  in  judging  we  make 
brightness  and  clearness  to  depend  on  contiguity,  as  it  ordinarily  does;  and 
as  the  star  is  bright,  we  unconsciously  think  it  near  ua 
Wh  do  th  su  ^n  consecluence  °f  terrestrial  objects  being  placed  in  close 
and  moon  ap-  comparison,  the  sun  and  moon  appear  larger  at  their  rising 
when  risinglnd  anc*  se^mo  than  at  any  other  time.  This  illusion  is  wholly  a 
setting  than  at  mental  one,  since  the  organs  of  vision  do  not  present  to  us  a 
larger  image  of  the  sun  or  moon  in  the  horizon  than  when  in 
the  zenith,  or  overhead. 

Wh         B  th  ^^e  moon?  ^though  a  sphere,  appears  to  be  a  flat  surface, 

moon,  a  sphere,  since  it  is  so  remote  that  we  are  unable  to  distinguish  any 
flat  surface  ?  *  difference  between  the  length  of  the  rays  reflected  from  the 

circumference,  and  those  reflected  from  the  center. 

Thus  the  rays  A  D  and  C  D,  Fig.  2  90,  appear  to  be  no  longer  than  the  ray 
2go  B  D  ;  but  if  all  the  rays  seem 

of  the  same  length,  the  part  B 
n°t  seem  to  be  nearer  to 
us  than  A  and  C  ;  and  there- 
fore the  curve  ABC  will  look 
like  a  flat,  or  horizontal  surface.  The  rays  A  D  and  C  D  are  240,000  miles 
long.  The  ray  B  D  is  238,910  miles  long. 

What  tw.0  715.  In  order  that  the  eye  may  see  distinctly, 
peSforedisI  the  picture  formed  upon  the  retina  must  be 
tiuct  vision?  illuminated  to  the  right  degree,  and  it  must 
also  remain  sufficiently  long  upon  the  retina  to  produce  a 
sensation  upon  the  optic  nerve. 

The  image-  of  an  object  on  the  retina  may  be  illuminated  too  much  or  too 
little  to  produce  a  sensible  perception  of  its  form.  Thus,  we  can  gain  no  idea 
of  the  form  of  the  sun  by  viewing  it  in  the  clear  sky,  because  the  degree  of 
illumination  is  so  great,  that  the  sense  of  vision  is  overpowered,  just  as  sounds 
are  sometimes  so  intense  as  to  be  deafening.  That  it  is  the  intense  splendor 
alone  which  prevents  a  distinct  perception  of  the  sun's  figure,  is  rendered 


358  WELLS'S  NATURAL  PHILOSOPHY. 

evident  by  the  fact  that  when  a  portion  of  the  light  is  cut  off  by  a  colored 
glass,  or  a  thin  cloud,  the  image  of  the  sun  is  seen  distinctly.  On  the  con- 
trary, we  fail  to  perceive  many  stars  at  night,  because  the  images  they  pro- 
duce on  the  retina  are  too  faintly  illuminated  to  produce  sensation.  That 
some  light  from  such  stars  actually  enters  the  eye,  is  proved  by  the  fact  that 
if  we  place  a  lens  before  the  eye,  and  collect  a  greater  quantity  of  their  light 
upon  the  retina,  they  at  once  become  visible. 

can  the  eye  ^he  eye  possesses  a  limited  power  of  accom- 
SegJees^fm^  modating  itself  to  various  degrees  of  illumi- 
nation? nation.  In  the  dark,  the  pupil  of  the  eye 
enlarges  its  opening,  and  allows  a  greater  number  of  rays 
to  fall  upon  the  retina  ;  in  the  light,  the  pupil  contracts 
in  proportion  to  the  intensity  of  the  illumination,  and 
diminishes  the  number  of  rays  falling  upon  the  retina. 

^^    .       .  This  change  does  not  take  place  instantaneously.     "When 

from 'the  light  we  leave  a  brilliantly  illuminated  apartment  at  night  and  go 
doVe^fiiKt  kto  tlie  dark  street»  we  are  unable  for  a  few  moments  to  see 
difficult  at  first  any  thing  distinctly.  The  reason  of  this  is,  that  the  pupil  of 
thingT6  any  ^e  eJe»  which  has  become  contracted  in  the  light,  is  unable 
to  collect  sufficient  rays  from  the  objects  in  the  dark  to  see 
them  distinctly.  In  a  few  moments,  however,  the  pupil  dilates,  allows  more 
rays  to  pass  through  its  aperture,  and  we  see  more  distinctly.  The  reverse 
of  this  takes  place  when  we  go  from  the  dark  into  the  light.  Cats,  owls,  and 
some  other  animals  are  able  to  see  distinctly  in  the  dark,  because  they  have 
the  power  of  enlarging  the  pupils  of  their  eyes  so  as  to  collect  the  scattered 
rays  of  light. 

Every  impression  made  by  light  remains  for  a  certain  length  of  time  on 
the  retina  of  the  eye,  according  to  the  intensity  of  its  effects,  and  a  measur- 
able period  is  necessary  to  produce  a  sensation. 

"We  are  unable,  when  riding  rapidly  on  a  railroad,  to  count 
prove  the  con-  the  posts  of  an  adjoining  fence,  because  the  light  from  each 
ima  TIT  on  th*6  Pos*  ^s  uPon  ^e  eye  in  such  rapid  succession,  that  the  dif- 
retina  after  the  ferent  images  become  confused  and  blended,  and  wo  do  not 
upie^red  ?  diS~  obtaia  a  distinct  vision  of  the  particular  parts. 

If  we  rotate  a  stick,  lighted  at  one  end,  somewhat  rapidly, 
it  seems  to  produce  a  complete  circle  of  fire ;  the  reason  of  this  is,  that  the 
eye  retains  the  image  of  any  bright  object  for  some  little  time  after  the  object 
is  withdrawn ;  and  as  the  light  of  the  stick  returns  to  each  particular  point  of 
its  path  before  the  image  previously  formed  has  faded  from  the  retina,  it  seems 
to  form  a  complete  circle  of  fire. 

Wh  is  it  not  ™3  continuance  of  tbe  impression  of  external  objects  on 
dark  when  we  the  retina  after  the  light  proceeding  from  them  has  ceased  to 
wink  ?  act,  is  the  reason  also  why  wo  are  not  sensible  of  darkness 

when  we  wink. 


THE  EYE,   AND   THE  PHENOMENA  OF  VISION.        359 

The  apparent  motion  of  certain  colored  figures  in  worsted  work,  known  by 
the  name  of  the  "  dancing  mice;"  is  due  to  the  fact  that  when  the  surface 
is  moved  in  a  particular  direction,  as  from  side  to  side,  the  impression  of  the 
color  on  the  retina  remains  for  an  appreciable  interval  after  the  figures  have 
moved,  and  this  gives  to  them  an  apparent  motion.  This  effect  will  not, 
however,  take  place  unless  the  colors  of  the  figures  and  the  ground- work  are 
very  brilliant  and  complementary  of  each  other,  as  red  upon  a  green  ground. 

716.  No  motion  is  perceptible  to  the  eye 
has  a  less  apparent  velocity  than  one 
degree  per  minute. 

It  is  for  this  reason  that  the  motions  of  the  heavenly  bodies  are  invisible,  not- 
withstanding their  immense  velocity.  The  apparent  motion  of  the  sun,  moon, 
and  stars,  owing  to  the  revolution  of  the  earth,  is  one  quarter  of  a  degree  a 
minute ;  but  if  the  earth  revolved  on  its  axis  in  six  hours  instead  of  twenty- 
four,  then  the  celestial  bodies  would  have  a  motion  of  one  degree  per  minute, 
and  their  movements  would  be  distinctly  perceptible. 

For  the  same  reason,  the  motions  of  the  hands  of  a  clock  are  not  per- 
ceptible to  the  eye. 

On  the  contrary,  when  a  body  moves  with  such  rapidity  from  one  position 
to  another,  that  its  image  does  not  remain  long  enough  upon  one  point  of  the 
retina  to  sufficiently  impress  it,  it  becomes  invisible.  Hence  it  is  that  a 
ball  discharged  from  a  cannon,  and  passing  transversely  across  the  eye,  is  not 
seen. 

HOW  is  appa-  Apparent  motion  is  affected  by  distance,  and 
the  motion  of  a  body  which  is  visible  at  one 
distance  may  be  invisible  at  another,  inasmuch 

as  the  angular  velocity  will  be  increased  as  the  distance  is 

diminished. 

Thus,  if  an  object  at  a  distance  of  57^-  feet  from  the  eye  move  at  the  rate 
of  a  foot  per  second,  it  will  appear  to  move  at  the  rate  of  one  degree  per 
second,  inasmuch  as  a  line  one  foot  long  at  57-J-  feet  distance  subtends  an 
angle  of  one  degree.  Now  if  the  eye  be  removed  from  such  an  object  to  a 
distance  of  115  feet,  the  apparent  motion  will  be  half  a  degree,  or  thirty  min- 
utes per  second;  and  if  it  be  removed  to  thirty  times  that  distance,  the  ap- 
parent motion  will  be  thirty  times  slower.  Or  if,  on  the  other  hand,  the  eyo 
be  brought  nearer  to  the  object,  the  apparent  motion  will  be  accelerated  in 
exactly  the  same  proportion  as  the  distance  of  the  eye  is  diminished. 

A  cannon-ball  moving  at  1,000  miles  an  hour  transversely  to  the  line  of 
yision,  and  at  a  distance  of  fifty  yards  from  the  eye,  will  be  invisible,  since  it 
will  not  remain  a  sufficient  time  in  any  one  position  to  produce  perception. 
The  moon,  however,  moving  with  more  than  double  the  velocity  of  the  can- 
non-ball, being  at  a  distance  of  240,000  miles,  has  an  apparent  motion  so  slow 
as  to  be  imperceptible  to  the  unassisted  eye. 


360 


WELLS'S   NATURAL   PHILOSOPHY. 


Describe  the 
portable  cam- 
era obscura, 

Fig.  291. 


SECTION     Y. 

OPTICAL     INSTRUMENTS. 

717.  The  portable  camera  obscura,  such  as  is  ordinarily 
used  for  photographic  purposes,  consists  of  a  pair  of  achro- 
matic double  convex  lenses,  set  in  a  brass  mounting  (see  Fig. 
291),  into  a  box  consisting  of  two  parts,  one  of  which 
slides  within  the  other.     The  total  length  of  the  box  is 
adjusted  to  suit  the  focal  distance  of  the  lens.     In  the 
back  of  the  box,  which  can  be  opened,  there  is  a  square 
piece  of  ground  glass  which  receives  the  images  of  the 
objects  to  which  tho  lens  is  directed,  and  by  sliding  the 
movable  part  of  the  box  in  or  out,  the 

ground  glass  can  be  brought  to  the  FIG.  292. 

precise  focus.  The  interior  of  the  box 
is  blackened  all  over  to  extinguish 
any  stray  light. 

The  appearance  of  the  camera  as 
described  is  represented  by  Fig.  292. 

What  are  Spec-          718.     Specta- 

cles  consist  of 
two  glass  or  crystal  lenses, 
of  such  a  character  as  to 

remedy  the  defects  of  vision  in  imperfect  eyes, — mounted 
in  a  frame  so  as  to  be  conveniently  supported  before  the 
eyes. 

Spectacles  are  of  two  kinds,  namely  those 

What  are  the  r  * 

two  varieties    with  convex  glasses,  which  magnify  objects, 

of  spectacles  ?  .  ...  o       J         J 

or  bring  their  images  nearer  to  the  eyes  ; 
and  those  with  concave  glasses,  which  diminish  the  ap- 
parent size  of  objects,  or  extend  the  limits  of  distinct 
vision. 

Some  persons,  in  order  to  protect  the  eye  from  excessive  light,  use  blue 
glasses  as  spectacles ;  they  are,  however,  more  mischievous  than  useful,  since 
they  absorb  different  parts  of  the  spectrum  unequally,  and  transmit  the  violet 
and  blue  rays. 

what  is  a  Mi-        ?19.  A  Microscope  is  any  instrument  which 

croscope?       magnifies  the  images  of  minute  objects,  and 

enables  us  to  see  them  with  greater  distinctness.     This 

result  is  produced  by  enlarging  the  angle  of  vision  under 


OPTICAL   INSTRUMENTS. 


361 


which  the  object  is  seen  —  since  the  apparent  magnitude 
of  every  body  increases  or  diminishes  with  the  size  of  this 
angle. 

Microscopes  are  of  two  kinds  —  simple  and  compound. 

In  the  simple  microscope,  the  object  under 

What    are   the  .  f  f    '  .J, 

two    varieties   examination  is  viewed  directly,  either  by  a 

of  microscopes?  ,  ,  , 

simple  or  compound  converging  lens. 
In  the  compound  microscope,  an  optical  image  of  the 
object,  produced  upon  an  enlarged  scale,  is  thus  viewed. 

The  simple  microscope  is  generally  a  simple  convex  lens,  in  the  focus  of 

which  the  object  to  be  examined 
is  placed.  Little  spheres  of  glass, 
formed  by  melting  glass  threads 
in  the  flame  of  a  candle,  form 
very  powerful  microscopes. 

Fig.  293  represents  the  mag- 
nifying principle  of  the  micro- 
scope. An  eye  at  E  would  seo 
the  arrow  A  B,  under  the  visual 
angle  A  E  B  ;  but  when  tho 
lens,  F  F'  is  interposed,  it  ia 
seen  under  the  visual  angle  at 
A'  E  B',  and  hence  it  appears 


FIG.  293. 


much  enlarged,  as  shown  in  the  image  A'  B'. 

Fig.  294  represents  the  most  itn- 
proved  form  of  mounting  a  simple 
microscope.  A  horizontal  support, 
capable  of  being  elevated  or  depressed 
by  means  of  a  screw  and  ratch-work, 
D,  sustains  a  double-convex  lens,  A. 
The  object  to  be  viewed  is  placed 
upon  a  piece  of  glass,  C,  upon  a  stand- 
ard, B,  immediately  below  the  lens. 
As  it  is  desirable  that  the  object  to 
be  magnified  should  be  strongly- 
illuminated.  a  concave  mirror  of  glass, 
M,  is  placed  at  tho  base  of  tho  instru- 
ment, inclined  at  such  an  angle  as  to 
reflect  the  rays  of  light  which  fall 
upon  it  directly  upon  the  object. 

720.TheCom- 

P°und     Micro- 

Microscope  ? 


FIG.  294. 


mat 


the 


362 


WELLS'S   NATURAL   PHILOSOPHY. 


simple  form,  consists  of  two  lenses,  so  arranged  that 
the  second  lens  magnifies  the  image  formed  by  the  first 
lens,  or  simple  microscope.  In  this  way  the  image  of 
the  object  is  examined  by  the  eye,  and  not  the  object 
itself!  *•& 

The  first  of  these  lenses  is  called  the  object- 
glass,  or  objective,  since  it  is  always  directed 
immediately  to  the  object,  which  is  placed 
very  near  it ;  and  the  latter  the  eye-glass,  or 
eye-piece,  inasmuch  as  the  eye  of  the  observer  is  applied 
to  it  to  view  the  magnified  image  of  the  object. 

FIG.  295. 


How  are  the 
lenses  of  a 
compound  mi- 
croscope desig- 
nated •( 


Fig.  295  illustrates  the  magnifying  principle  of  the  compound  microscope. 
O  represents  the  object-glass  placed  near  the  object  to  be  viewed,  A  B,  and 
G-,  the  eye-glass  placed  near  the  eye  of  the  observer,  E.  The  object-glass,  0, 
presents  a  magnified  and  inverted  image,  a  6,  of  the  object  at  the  focus  of  tho 
eye-glass,  G-.  The  image  thus  formed,  by  means  of  the  second  lens  or  eye- 
glass, G,  is  magnified  and  brought  to  the  eye  at  E,  so  as  to  appear  under  tho 
enlarged  visual  angle,  A'  E  B'.  If  we  suppose  the  object-glass,  0,  to  have  a 
magnifying  power  of  25 — that  is,  if  the  image  a  b  equals  25  A  B,  and  tho 
eye-glass,  G,  to  have  a  magnifying  power  of  4 — then  the  total  magnifying 
power  of  the  microscope  will  be  4  times  25,  or  100;  that  is  to  saj',  tho 
image  will  appear  100  times  the  size  of  the  object. 

Fig.  296  represents  the  most  approved  form  of  mounting  tho  lenses 
which  compose  a  compound  microscope.  Tho  tube,  A,  which  contains  in 
its  upper  part  the  eye-glass,  slides  into  another  tube,  B,  in  the  bottom  of 
which  the  object-glass  is  fixed ;  this  last  tube  also  moves  up  and  down  in 
the  stand,  C,  and  in  this  way  the  lenses  in  the  tubes  may  be  adjusted  to  tho 
proper  distance  from  each  other  and  the  object.  M  is  a  mirror  for  reflecting 
light  upon  the  object,  and  S  a  support  on  which  tho  object  to  be  examined 
to  placed. 


OPTICAL   INSTRUMENTS. 


363 


What   is    a 
Telescope  ? 


How     many 
kinds  of   tele- 
scopes         are 
there? 


What  is  a  Re- 
fracting Tele- 
scope ? 


B 


721.  A  Telescope  is  any  FI&  296. 
instrument  which  magni- 
fies and  renders  visible  to  the  eye  the 

images  of  distant  objects.  This  result 
is  effected  in  the  same  manner  as  in 
the  microscope,  viz ,  by  enlarging  the 
visual  angle  under  which  the  objects 
are  seen. 

Telescopes   are    of  two 

kinds,  refracting  telescopes 

and  reflecting  telescopes; 
the  principle  of  construction  in  both 
being  the  same  as  that  of  the  com- 
pound microscope. 

722.  The      Kefracting 
Telescope   consists    essen- 
tially of  two  convex  lenses, 

the  object-glass  and  the  eye-glass. 
An  inverted  image  of  an  object,  as  a 
star,  is  produced  by  the  object-glass, 
and  magnified  by  the  eye-glass. 

Fig.  297  represents  the  principle  of  construction 
of  the  astronomical  refracting  telescope.     0  is  an 

object-glass  placed  at  the  end  of  a  tube,  which  collects  the  rays  proceeding 
from  a  distant  object  and  forms  an  inverted  imago  of  the  same  at  o  of,  in  the 
focus  of  the  eye-glass,  G-.  By  this  the  image  is  magnified  and  viewed  by  the 
eye  at  E. 

FiG.  297. 


What  Is 
Equatorial 
Telescope  ? 


723.  When  a  telescope  is  mounted  on  an 
axis  inclined  to  the  latitude  of  a  place,  so  that 
it  can  follow  a  star,  or  planet,  in  its  diurnal 

revolution,  by  a  single  motion,  it  is  called  an  EQUATO- 

KIAL  TELESCOPE. 

Such  an  instrument  is  generally  moved  by  clock-work,  and  is  accurately 


364 


WELLS  S   NATURAL   PHILOSOPHY. 


counterbalanced  by  an  arrangement  of  weights.  A  small  telescope  called  tho 
finder,  is  attached  near  the  eye  end  of  the  large  one  ;  this  is  so  adjusted  that 
when  tho  object  is  seen  through  it,  it  appears  in  the  field  of  the  large  tele- 
scope, thus  saving  much  trouble  in  directing  the  instrument  toward  any  par- 
ticular object. 

The  mounting  and  attachments  of  an  equatorial  telescope  are  represented 
in  Fig.  298. 

Fin.  298. 


What  is  a  Spy- 
e-lass  ? 


724.  A  spy-glass,  or  terrestrial  telescope, 
differs  from  an  astronomical  telescope  only  in 
an  adjustment  of  lenses,  which  enables  the  observer  to  see 
the  images  of  objects  erect  instead  of  inverted.  This  is 
effected  by  the  addition  of  two  lenses  placed  between  the 
eye  and  the  image. 

The  arrangement  of  the  lenses,  and  tho  course  of  the -rays  of  light,  in  a 
common  spy-glass,  are  represented  in  Fig.  299.  0  is  the  object-glass,  and  C 
L  M  the  eye-glasses,  placed  at  distances  from  each  other  equal  to  double  their 
focal  length.  The  progress  of  the  rays  through  the  object-glass,  0,  and  the 
first  eye-glass,  C,  is  the  same  as  in  the  astronomical  telescope,  and  an  inverted 


OPTICAL   INSTBUMENTS. 

image  is  formed ;  but  the  second  lens,  L,  reverses  the  i 
therefore,  in  an  erect  position  by  the  last  eye-glass,  M. 

FIG.  299. 

c 


365 

,  which  is  viewed 


what  is  the 


725.  The  common  opera-glass,  also  called 
Se8trucopera-  tne  Galilean  telescope  from  Galileo,  its  in- 
giass?  ventor,  consists  of  a  single  convex  object-glass 

and  a  concave  eye-glass. 

Fig.  300  represents 
the  construction  of  this 
form  of  telescope.  0  is 
a  single  convex  object- 


FiG.  300. 


in  the   focus  of 
which  an  inverted  image 

of  the  object  would  be  naturally  formed,  were  it  not  for  the  interposition  of 
the  double-concave  lens,  E.  This  receiving  the  converging  rays  of  light, 
causes  them  to  diverge  and  enter  the  eye  parallel,  and  form  an  erect  image. 

what  is  a  Re-  726.  A  Reflecting  Telescope  consists  essen- 
wope"?8  Tele"  tially  of  a  concave  mirror,  the  image  in  which 
is  magnified  by  means  of  a  lens.  The  mirror 
employed  in  reflecting  telescopes  is  made  of  polished 
metal,  and  is  termed  a  speculum. 

The  manner  in  which  the  rays  of  light  falling  upon  the  concave  speculum 
of  a  reflecting  telescope  are  caused  to  converge  to  a  focus  is  clearly  shown 
in  Fig.  301.  The  image  formed  at  this  focus  is  viewed  through  a  double- 
convex  lens. 

FIG.  301. 


Fig.  302  represents  one  of  the  earliest  forms  of  the  reflecting  telescope,  called 
from  its  inventor,  Mr.  Gregory,  the  "  Gregorian  Telescope."  It  consists  of 
a  concave  metallic  speculum,  A  B,  with  a  hole  in  its  center,  and  a  convex 
eye-glass,  E,  the  whole  being  fitted  into  a  tube.  An  inverted  image,  n'  ra', 
of  a  distant  object  is  formed  by  the  speculum,  A  B ;  this  image  is  again 


366 


WELLS'S   NATURAL   PHILOSOPHY. 


N 


reflected  by  a  small 
mirror,  C  D,  and  forms 
an  erect  image  at  n  m, 
which  is  magnified  by 
the  lens,  E,  when  ob- 
served by  the -eye. 

Another  form  of 
the  reflecting  tele- 
scope, called  the 
"Newtonian,  is  rep- 
resented in  Fig.  303. 
It  consists  of  a  large 
concave  speculum, 
A  B,  set  in  one  end 

of  a  tube,  and  a  small  plane  mirror,  C  D,  placed  obliquely  to  the  axis  of  the 
tube.     The  image  of  a  distant  object  formed  by  the  speculum,  A  B  is  reflect- 
ed by  the  mirror,  C  Dr  to  a  point,  m'  n',  on  the  side  of  the  tube,  and  is  there 
yiewed  through,  an  eye-glass,  E. 
FiG.  304. 


Large  reflecting  telescopes, 
at  the  present  day,  are  so  con- 
structed as  to  dispense  with 
the  Email  mirror.  This  is  ac- 
complished by  slightly  inclin- 
ing the  large  speculum,  so  as 
to  throw  the  image  on  one 
side  where  it  is  viewed  by  an  eye-glass,  as  is  represented  in  Fig.  -304. 

FIG.  305. 


OPTICAL    INSTRUMENTS. 


367 


The  largest  telescope  ever  constructed  is  that  made  by  Lord  Rosso.  This 
instrument,  which  is  a  reflecting  'telescope,  is  located  at  Parsonstown,  in 
Ireland.  Its  external  appearance  and  method  of  mounting  is  represented  in 
Fig.  305.  The  diameter  of  the  speculum  is  6  feet,  and  its  weight  about  4  tons. 
The  tube  in  which  it  is  placed  is  of  wood  hooped  with  iron,  52  feet  in  length, 
and  7  feet  in  diameter.  It  is  counterpoised  in  every  direction,  and  moves 
between  two  walls,  24  feet  distant,  72  feet  long,  and  48  feet  high.  The  ob- 
server stands  on  a  platform  which  rises  or  falls,  or  at  great  elevation  upon 
sliding  galleries  which  draw  out  from  the  wall. 

This  telescope  commands  an  immense  field  of  vision,  and  it  is  said  that  ob- 
jects as  small  as  100  yards'  cube,  can  be  distinctly  observed  by  it  in  the  moon 
at  a  distance  of  240,000  miles.* 

what  is  a  Ma^c       727.  The  Magic  Lantern  is  an  optical  in- 
Lanteri,?       strument  adapted  for  exhibiting  pictures  paint- 
ed on  glass  in  transparent  colors,  on  a  large  scale,  by  means 
of  magnifying  lenses. 

FIG.  30?. 


It  consists  of  a  metallic  box,  or  lantern,  A  A',  Fig.  306,  containing  a  lamp, 
L,  behind  which  is  placed  a  metallic  concave  mirror,  p  q.  In  front  of  tho 
lamp  are  two  lenses,  fixed  in  a  tube  projecting  from  the  side  of  the  lantern, 
one  of  which,  m,  is  called  the  illuminator,  and  the  other  the  magnifier.  Tho 
objects  to  be  exhibited  are  painted  on  thin  plates  of  glass,  which  are  intro- 
duced by  a  narrow  opening  in  the  tube,  c  d,  between  the  two  lenses.  Tho 
mirror  and  the  first  lens,  ?/i,  serve  to  illuminate  the  painting  in  a  high  degree, 
for  the  lamp  being  placed  in  their  foci,  they  throw  a  brilliant  light  upon  it, 
and  the  magnifying  lens,  n,  which  can  slide  in  its  tube  a  little  backward  and 
forward,  is  placed  in  such  a  position  as  to  throw  a  highly  magnified  image  of 
the  drawing  upon  a  screen,  several  feet  off,  the  precise  focal  distance  being 
adjusted  by  sliding  the  lens.  The  further  the  lantern  is  withdrawn  from  the 

*  By  the  aid  of  this  mighty  instrument,  "  one  of  the  most  wonderful  contributions  of 
art  and  science  the  world  has  yet  seen,"  what  astronomers  have  before  called  nebula,  on 
account  of  their  cloud-like  appearance,  have  been  discovered  to  be  stars,  or  suns,  analo- 
gous, in  all  probability,  in  constitution,  to  our  own  sun.  In  the  constellations  Andro- 
meda and  the  sword-hilt  of  Orion,  both  of  which  are  visible  to  the  naked  eye,  these 
«l<Hid-like  patches  have  been  seen  as  clusters  of  stars. 


368 


WELLS'S   NATURAL   PHILOSOPHY. 


screen,  the  larger  the  iinaoce  will  appear ;  but  when  the  distance  is  considera- 
ble the  image  becomes  indistinct. 

what  are  Dis-  728.  The  beautiful  optical  combinations 
solving  views?  ]jnowil  as  Dissolving  Views  are  produced  by 
means  of  two  magic  lanterns  of  equal  power,  so  placed  as 
to  throw  pictures  of  precisely  equal  magnitude  on  the 
same  part  of  the  same  screen.  By  gradually  closing 
the  aperture  of  one  lantern  and  opening  that  of  the  other, 
a  picture  formed  by  the  first  may  seem  to  be  dissolved 
away  and  changed  into  another. 

Thus,  if  the  picture  produced  by  one  lantern  represents  a  day  landscape, 
and  the  picture  produced  by  the  other  the  same  landscape  by  night,  the  one 
may  be  changed  into  the  other  so  gradually  as  to  imitate  with  great  exactness 
the  appearance  of  approaching  night. 

what  is  a  soiar        729.  The  S$lar  Microscope  is  an  optical  in- 
Microscope?     gtrument  constructed  on  the  principle  of  the 
magic  lantern,  but  the  light  which  illuminates  the  object 
is  supplied  by  the  sun  instead  of  a  lamp. 

This  result  is  effected  by.  admitting  the  rays  of  the  sun  into  a  darkened 
room,  through  a  lens  placed  in  an  aperture  in  a  window  shutter,  the  rays 

being  received  by  a  plane  mirror  fixed  ob- 
liquely, outside  the  shutter,  and  thrown 
horizontally  on  the  lens.  The  object  is 
placed  between  this  lens  and  another 
smaller  lens,  as  in  the  magic  lantern ;  and 
the  magnified  image  formed  is  received  upon 
a  screen.  In  Fig.  307,  which  represents 
the  construction  of  the  solar  microscope,  C 
is  a  plane  mirror,  A  the  illuminating  lens, 
and  B  the  magnifying  lens.  The  objects  to 

be  magnified  are  placed  between  the  lenses  A  and  B.  In  consequence  of 
the  superior  illumination  of  the  object  by  the  rays  of  the  sun,  it  will  bear  to 
be  magnified  much  more  highly  than  with  the  lantern.  Hence  this  form  of 
microscope  is  often  employed  to  represent,  on  a  very  enlarged  scale,  various 
minute  natural  objects,  such  as  animalcule  existing  in  various  liquids,  crys- 
tallization of  various  salts,  and  the  structure  of  vegetable  substances. 


CHAPTER    XV. 

Jit'.'. 

ELECTRICITY. 

Eiec-  730.  ELECTRICITY  is  one  of  those  subtle 
tricity?  agents  without  weight,  or  form,  that  appear  to 
be  diffused  through  all  nature,  existing  in  all  substances 
without  affecting  their  volume  or  their  temperature,  or 
giving  any  indication  of  its  presence  when  in  a  latent,  or 
ordinary  state.  When,  however,  it  is  liberated  from  this 
repose,  it  is  capable  of  producing  the  most  sudden  and 
destructive  effects,  or  of  exerting  powerful  influences  by 
a  quiet  and  long-continued  action. 

HO*  may  eiec-        731.  Electricity  may  be  excited,  or  called 
cited?  be  ex"    iQto  activity  by  mechanical  action,  by  chemical 
action,  by  heat,  and  by  magnetic  influence. 

"We  do  not  know  any  reason  why  the  means  above  enumerated  should  de- 
velop electricity  from  its  latent  condition,  neither  do  we  know  whether  elec- 
tricity is  a  material  substance,  a  property  of  matter,  or  the  vibration  of  an 
ether.  The  general  opinion  at  the  present  day  is  that  electricity,  like  light 
and  heat  is  the  result  of  vibrations  of  an  ether  pervading  all  space. 

is  eiec-  732.  The  most  ordinary  and  the  easiest  way 
°f  exciting  electricity  is  by  mechanical  action 
— by  friction. 

HOW  does  eiec-        If  we  rub  a  glass  rod,  or  a  piece  of  sealing- 
tncity  excited    wax^  or  resm?  or  amber,  with  a  dry  woolen,  or 

silk  substance,  .these  substances  will  imme- 
diately acquire  the  property  of  attracting  light  bodies, 
such  as  bits  of  paper,  silk,  gold-leaf,  balls  of  pith,  etc. 

This  attractive  force  is  so  great,  that  even  at  the  dis- 
tance of  more  than  a  foot,  light  substances  are  drawn  to- 
ward the  attracting  body.  The  cause  of  this  attraction 
is  called  electricity. 

Thales,  one  of  the  seven  wise  men  of  Greece,  noticed  and  recorded  the 
fact  more  than  two  thousand  years  ago,  that  amber  when  rubbed  would  at- 

16* 


370 


WELLS'S  NATURAL  PHILOSOPHY. 


What  other  ef- 
fects beside  at- 
traction are  no- 
ticed in  exciting 
electricity  by 
friction  ? 


When  is  a  body 
said  to  be  elec- 
trified? 


What  is  electric 
attraction  ? 


What  is  electric 


tract  light  bodies  ;  and  the  name  electricity,  used  to  designate  such  pheno- 
mena has  been  derived  from  the  Greek  word  jyAe/crpov,  electron,  signifying 
amber. 

If  the  friction  of  the  glass,  wax,  amber,  etc., 
is  vigorous,  small  streams  of  light  will  be  seen, 
a  crackling  noise  heard,  and  sometimes  a  re- 
markable odor  will  be  perceived. 

733.  When,  by  friction  or  other  means,  elec- 
tricity is  developed  in  a  body,  it  is  said  to  be 
electrified,  or  electrically  excited. 

The  tendency  which  an  electrified  body  has 
to  move  toward  other  bodies,  or  of  other  bodies 
toward  it,  is  ascribed  to  a  force  called  electric  attraction. 

Every  electrified  body,  in  addition  to  its  at- 
tractive  force,  manifests  also  a  repulsive  force. 
This  is  proved  by  the  fact  that  light  substances,  after 
touching  an  electrified  body,  recede  from  it  just  as  actively 
as  they  approached  it  before  contact.  Such  action  is  as- 
cribed to  a  force  called  electric  repulsion. 

Thus,  if  we  take  a  dry  glass  rod,  rub  it  well 
with  silk,  and  present  it  to  a  light  pith  ball,  or 
feather,  P,  suspended  from  a  support  by  a  silk 
thread,  the  ball  or  feather  will  be  attracted  to- 
ward the  glass,  as  seen  at  G,  Fig.  308.  After  it 
has  adhered  to  it  a  moment,  it  will  fly  off,  or  be 
repelled,  as  P'  from  G'. 

The  same  thing  will  happen  if  sealing-wax  be 
rubbed  with  dry  flannel,  and  a  like  experiment 
made  ;  but  with  this  remarkable  difference,  that 

when  the  glass  repels  the  ball,  the  sealing-wax  attracts  it,       -plQ  ^QQ 
and  when  the  wax  repels,  the  glass  will  attract.     Thus  if  we  . 

suspend  a  light  pith  ball,  or  feather,  by  a  silk  thread,  as  in 
Fig.  309,  and  present  a  stick  of  excited  sealing-wax,  S,  on 
one  side,  and  a  tube  of  excited  glass,  G,  on  the  other,  the  ball 
will  commence  vibrating  like  a  pendulum  from  one  to  the 
other,  being  alternately  attracted  and  repelled  by  each,  the 
one  attracting  when  the  other  repels;  hence  we  conclude 
that  the  electricities  excited  in  the  glass  and  wax  are  different. 

734.  As  the  electricity  developed 

Is  there  more        _.  /»      i  I-ITI  i 

than  one  kind    friction  of  glass  and  other  like  substances  is 
essentially  different  from  that  developed  by 


FIG.  308. 


by  the 


of  electricity  ? 


ELECTRICITY.  371 

the  friction  of  resin,  wax,  etc.,  it  has  been  inferred  that 
there  are  two  kinds  or  states  of  electricity — the  one  called 
vitreous,  because  especially  developed  on  glass,  and  the 
other  resinous,  because  first  noticed  on  resinous  sub- 
stances. 

what  is  the  The  fundamental  law  which  governs  the  re- 
-f  lation  of  these  two  electricities  to  each  other, 
d  and  which  constitutes  the  basis  of  this  depart- 
ment of  physical  science,  may  be  expressed  as 
follows  : — 

Like  electricities  repel  each  other,  unlike  electricities 
attract  each  other. 

Thus,  if  two  substances  are  charged  with  vitreous  electricity,  they  repel 
each  other;  two  substances  charged  with  resinous  electricity  also  repel  each 
other ;  but  if  one  is  charged  with  vitreous,  and  the  other  with  resinous  elec- 
tricity, they  attract  each  other. 

when  is  a  body  ^35.  When  a  body  holds  its  own  natural 
non-electrified?  quantity  of  electricity  undisturbed,  it  is  said 
to  be  non-electrified. 

When  an  electrified  body  touches  one  that 
trined  *n  body  is  non-electrified,  the  electricity  contained  in 
n0oa%feetriftedt  the  former  is  transferred  in  part  to  the  latter. 

Thus,  on  touching  the  end  of  a  suspended  silk  thread  with  a 
piece  of  excited  wax  or  glass,  electricity  will  pass  from  the  wax  or  glass  into 
the  silk,  and  render  it  electrified ;  and  the  silk  will  exhibit  the  effects  of  the 
electricity  imparted  to  it,  by  moving  toward  any  object  that  may  be  placed 
near  it. 

736.  Two  theories,  based  upon  the  phenom- 

Whattwothe-  '  f.  * 

ones  have  been  ena  of  attraction  and  repulsion,  have  been 
count  for  eiec-  formed  to  account  for  the  nature  and  origin  of 

trical action?  •,       .    .    ..  m,  i  .  , 

electricity.  These  two  theories  are  known  as 
the  theory  of  two  fluids,  and  the  theory  of  the  single  fluid ; 
or  the  theory  of  Du  Fay,  an  eminent  French  electrician, 
and  the  theory  of  Dr.  Franklin. 

737.  The  theory  of  two  fluids,  or  the  theory 

What    is    the          /.  TX       -B,  ,,  n    i      v          •       ,1      - 

theory  of  two     ot  Du  i  ay,  supposes  that  all  bodies,  in  their 

fluids?  i     i    I  j     3    -u  j-       i 

natural  state,  are  pervaded  by  an  exceedingly 
thin  subtle  fluid,  which  is  composed  of  two  constituents, 


372  WELLS'S   NATUKAL   PHILOSOPHY. 

or  elements,  viz.,  the  vitreous  and  the  resinous  electrici- 
ties. Each  kind  is  supposed  to  repel  its  own  particles,  but 
attract  the  particles  of  the  other  kind. 

"When  these  two  fluids  pervade  a  body  in  equal  quantities,  they  neutralize 
each  other  in  virtue  of  their  mutual  attraction,  and  remain  in  repose ;  but 
when  a  body  contains  more  of  one  than  of  the  other,  it  exhibits  vitreous  or 
resinous  electricity,  as  the  case  may  be. 

738.  The  theory  of  a  single  fluid,  or  the 
theory  of   a     theory  propounded  by  Dr.  Franklin,  supposes 

single  fluid?  .          J  .  *       r         f          .    J  „     .  j        *  l, 

the  existence  of  a  single  subtile  fluid,  without 
weight,  equally  distributed  throughout  nature  ;  every  sub- 
stance being  so  constituted  as  to  retain  a  certain  quantity, 
which  is  necessary  to  its  physical  condition. 

When  a  substance  pervaded  by  this  single  fluid  is  in  its  natural  state  or 
condition,  it  offers  no  evidence  of  the  presence  of  electricity;  but  when  its 
natural  condition  is  disturbed  it  appears  electrified.  The  difference  between 
the  electricity  developed  by  glass  and  that  by  resin  is  explained  by  this 
theory,  by  supposing  electrcal  excitation  to  arise  from  the  difference  in  the 
relative  quantities  of  this  principle  existing  in  the  body  rubbed  and  the  rub- 
ber, or  in  their  powers  of  receiving  and  retaining  electricity.  Thus  one  body 
becomes  overcharged  by  having  abstracted  this  principle  from  the  other. 

whatareposi-  739.  The  two  different  conditions  of  electric- 
tire  aefecntrfd-  ^7>  which  were  called  by  Du  Fay  vitreous  and 
11687  resinous  electricities,  were  designated  by  Dr. 

Franklin  as  positive  and  negative,  or  plus  and  minus. 
Thus  a  body  which  has  an  overplus  of  electricity  is  called 
positive,  and  one  that  has  less  than  its  natural  quantity 
is  called  negative^ 

The  theory  of  a  single  fluid  has,  until  quite  recently,  been  generally  adopted 
by  scientific  men,  and  the  terms  positive  and  negative  electricities  are  uni- 
versally used  in  the  place  of  vitreous  and  resinous.  Within  the  last  few 
years,  however,  some  discoveries  have  been  made  which  seem  to  indicate 
that  the  theory  of  two  fluids  is  the  one  which  approaches  nearest  to  the  truth. 
What  is  Pro  "^n  adcu'tion  to  these  two  theories  respecting  the  nature  of 
fessor  Fara-  electricity,  another  has  been  proposed  by  Professor  Faraday, 
JStrfdtyY  °f  of  Enoland-  He  considers  electricity  to  be  an  attribute,  or 
quality  of  matter,  like  what  we  conceive  of  the  attraction  of 
gravitation.* 

*  It  is  not  easy  to  perfectly  explain  to  a  beginner  the  view  which  has  been  taken  by 
Professor  Faraday  (who  is  at  present  the  highest  recognized  authority  on  this  subject)  re- 
specting the  nature  of  electricity.  The  following  statement,  as  given  by  a  late  writer 
(Robert  Hunt),  may  be  sufficiently  comprehensive  and  clear  :  "  Every  atom  of  matter  is 


ELECTRICITY.  373 

••      .  740.  Light,  heat,  and  electricity  appear  to  have  some  prop- 

connection  be-  erties  in  common,  and. each  may  be  made,  under  certain  cir- 
heatnand1eleo'  cumstances>  to  produce  or  excite  the  other.  All  are  so  light, 
tricity  ?  subtle,  and  diffusive,  that  it  has  been  found  impossible  to  recog- 

nize in  them  the  ordinary  characteristics  of  matter.    Some  sup- 
pose  that  light,  heat,  and  electricity  are  all  modifications  of  a  common  principle. 
741.  Electricity  exists  in,  or  may  be  excited  in  all  bodies, 
electrical     di-     There  are  no  exceptions  to  this  rule,  but  electricity  is  de- 
Bubstanees  ? aU     vel°Ped  in  some  Bodies  with  great  ease,  and  in  others  with 
great  difficulty.     All  substances,  therefore,  have  been  divided 
into  two  classes,  viz.,  Electrics,  or  those  which  can  be  easily  excited,  and 
Non-electrics,  or  those  which  are  excited  with  difficulty.     Such  a  division  is, 
however,  of  little  practical  value  in  science,  and  at  present  is  not  generally 
recognized. 

There  is  no  certain  test  which  will  enable  us  to  determine,  previous  to  ex- 
periment, which  of  two  bodies  submitted  to  friction  will  produce  positive,  and 
which  negative  electricity.  Of  all  known  substances,  a  cat's  fur  is  the  most 
susceptible  of  positive,  and  sulphur  of  negative  electricity.  Between  these 
extreme  substances  others  might  be  so  arranged,  that  any  substance  in  the 
list  being  rubbed  upon  any  other,  that  which  holds  the  highest  place  will  be 
positively  electrified,  and  that  which  holds  the  lower  place  negatively  elec- 
trified. For  instance,  smooth  glass  becomes  positively  electrified  when  rub- 
bed with  silk  or  flannel,  but  negatively  electrified  when  excited  by  the  back 
of  a  living  cat.  Sealing-wax  becomes  positive  when  rubbed  with  the  metals, 
but  negative  by  any  thing  else. 

can  one  eiec-  In  no  case  can  electricity  of  one  kind  be 
Ste"7  without  excited  without  setting  free  a  corresponding 
settmg  free  the  amount  of  electricity  of  the  other  kind  ;  hence, 
when  electricity  is  excited  by  friction,  the  rub- 
ber always  exhibits  the  one,  and  the  electric,  or  body 
rubbed,  the  other. 

what  are  con-  742.  Bodies  differ  greatly  in  the  freedom 
non-conductor's  w^n  which  they  allow  electricity  to  pass  over 
of  electricity?  or  through  them.  Those  substances  which 

regarded  as  existing  by  virtue  of  certain  properties  or  powers,  these  being  merely  pecu- 
liar affections,  which  may  be  regarded  as  being  of  a  similar  nature  to  vibrations.  It  is 
assumed  that  the  electric  state  is  but  a  mode  or  form  of  one  of  these  affections.  One  par- 
ticle of  matter,  having  received  this  form  of  disturbance,  communicates  it  to  all  contigu- 
ous particles — that  is,  those  which  are  next  to  it,  although  not  in  contact — and  this  com- 
munication of  force  takes  place  more  or  less  readily,  the  communicating  particles  assuming 
a  polarized  state— which  may  be  explained  as  a  state  presenting  two  dissimilar  extremities. 
When  the  communication  is  slow,  the  polarized  state  is  highest,  and  the  body  is  said  to 
be  an  insulator  t  insulation  being  the  result.  If  the  particles  communicate  their  condition 
readily,  they  are  termed  conductors :  conduction  is  the  result.  The  phenomena  of  in- 
duction, or  the  production  of  like  effects  in  contiguous  Bodies,  is,  therefore,  according  to 
this  view,  but  something  analogous  to  the  communication  of  tremors,  or  vibrations." 


374  WELLS'S   NATURAL   PHILOSOPHY. 

facilitate  its  passage  are  called  conductors  ;  those  that  re- 
tard, or  almost  prevent  it,  are  called  non-conductors. 

No  substance  can  entirely  prevent  the  passage  of  electricity,  nor  is  there 
any  which  does  not  oppose  some  resistance  to  its  passage. 

what  sub-  Of  all  bodies,  the  metals  are  the  most  per- 
c?nndcuec8torre8g<jfd  fect  conductors  of  electricity  ;  charcoal,  the 
electricity?  earth,  water,  moist  air,  most  liquids,  except 
oils,  and  the  human  body,  are  also  good  conductors  of 
electricity. 

what  IB  the  ?43.  The  velocity  with  which  electricity 
Jridty?  ***"  passes  through  good  conductors  is  so  great, 
that  the  most  rapid  motion  produced  by  art 
appears  to  be  actual  rest  when  compared  to  it.  Some 
authorities  have  estimated  that  electricity  will  pass 
through  copper  wire  at  the  rate  of  two  hundred  and 
eighty-eight  thousand  miles  in  a  second  of  time — a  ve- 
locity greater  than  that  of  light.  The  results  obtained, 
however,  by  the  United  States  Coast  Survey,  with  iron 
wire,  show  a  velocity  of  from  15,000  to  20,000  miles  per 
second. 

what  snb-  Gum  shellac  and  gutta  percha  are  the  most 
conStorlTf  perfect  non-conductors  of  electricity  ;  sulphur, 
electricity?  sealing- wax,  resin,  and  all  resinous  bodies, 
glass,  silk,  feathers,  hair,  dry  wool,  dry  air,  and  baked 
wood  are  also  non-conductors. 

Electricity  always  passes  by  preference  over  the  best 
conductors. 

Thus,  if  a  metallic  chain  or  wire  is  held  in  the  hand,  one  end  touching  the 
ground  and  the  other  brought  into  contact  with  an  electrified  body,  no  part 
of  the  electricity  will  pass  into  the  hand,  the  chain  being  a  better  conductor 
than  the  flesh  of  the  hand.  But  if,  while  one  end  of  the  chain  is  in  contact 
with  the  conductor,  the  other  be  separated  from  the  ground,  then  the  electricity 
will  pass  into  the  hand,  and  will  be  rendered  sensible  by  a  convulsive  shock. 

whenisabody        744.  When  a  conductor  of  electricity  is  sur- 
rounded on  all  sides  by  non-conducting  sub- 
stances, it  is  said  to  be  insulated;  and  the  non-conducting 
substances  which  surround  it  are  called  insulators. 


ELECTRICITY.  375 

When   a   conducting  body  is  insulated,  ifc 
ith    retains  upon  its  surface  the  electricity  com- 
niumcated  to  it,  and  in  this  condition  it  is 
said  to  be.  charged  with  electricity. 

A  conductor  of  electricity  can  only  remain  electric  as  long  as  it  is  insulated, 
that  is,  surrounded  by  perfect  non-conductors.  The  air  is  an  insulator,  since, 
if  it  were  not  so,  electricity  would  be  instantly  withdrawn  by  the  atmosphere 
from  electrified  substances.  Water  and  steam  are  good  conductors,  conse- 
quently, when  the  atmosphere  is  damp,  the  electricity  will  soon  be  lost, 
which,  in  a  dry  condition  of  the  air,  would  have  adhered  to  an  insulated  con- 
ductor for  a  long  period  of  time. 

Thus  a  globe  of  metal  supported  on  a  glass  pillar,  or  suspended  by  a  silken 
cord,  and  charged  with  electricity,  will  retain  the  charge.  If,  on  the  con- 
trary, it  were  supported  on  a  metallic  pillar,  or  suspended  by  a  metallic  wire, 
the  electricity  would  immediately  pass  away  over  the  metallic  surface  and 
escape. 

In  the  experiments  made  with  the  pith  balls  (§  733,  Fig.  308),  the  silk 
thread  by  which  they  were  suspended  acts  as  an  insulator,  and  the  electricity 
with  which  they  become  charged  is  not  able  to  escape. 

745.  When  electricity  is  communicated  to 

Does  elcctrici-  .  " 

ty  accumulate    a  conducting  body  it  resides  merely  upon  the 

upon  the  stir-  11  J       I 

face  or  the  in-    surface.  and  does  not  penetrate  to  any  depth 

terior  of  bodies?         .    ,  .  r  J          r 

within. 

Thus,  if  a  solid  globe  of  metal  suspended  by  a 
silken  thread,  or  supported  upon  an  insulated 
glass  pillar,  be  highly  electrified,  and  two  thin 
hollow  caps  of  tin-foil  or  gilt  paper,  furnished 
with  insulating  handles,  as  is  represented  in 
Fig.  310,  be  applied  to  it,  and  then  withdrawn, 
it  will  be  found  that  the  electricity  has  been 
completely  taken  off  the  sphere  by  means  of  the  caps. 

An  insulated  hollow  ball,  however  thin  its  substance,  will  contain  a  charge 
of  electricity  equal  to  that  of  a  solid  ball  of  the  same  size,  all  the  electricity  in 
both  cases  being  distributed  upon  the  surface  alone. 

In  the  case  of  a  spherical  body  charged  with  electricity, 
form  o^sTbody     *ne  distribution  is  equal  all  over  the  surface  ;    but  when  the 


electrical      ***     ^°^  *°  which  the  electricity  is  communicated  is  larger  in  one 
dition  ?  "    "direction  than  the  other,  the  electricity  is  chiefly  found  at  its 

longer  extremities,  and  the  quantity  at  any  point  of  its  sur- 
face is  proportional  to  its  distance  from  the  center. 

The  shape  of  a  body  also  exercises  great  influence  in  retaining  electricity  : 
it  is  more  easily  retained  by  a  sphere  than  by  a  spheroid  or  cylinder;  but  it- 
readily  escapes  from  a  point,  and  a  pointed  object  also  receives  it  with  the 
greatest  facility. 


376  WELLS'S  NATURAL  PHILOSOPHY. 

What  is  the  ^6.  The  earth  is  considered  as  the  great 
SS^SiJ0?  Seneral  reservoir  of  electricity. 

When  by  means  of  a  conducting  substance  a  communi- 
cation is  established  between  a  body  containing  an  excess  of  electricity  and 
the  earth,  the  body  will  immediately  lose  its  surplus  quantity,  which  passes 
into  the  earth  and  is  lost  by  diffusion. 

mat  is  eiec-  747.  When  a  body  charged  with  electricity 
tricaiinduction?  0£  one  kind  fe  brought  into  proximity  with 

other  bodies,  it  is  able  to  induce  or  excite  in  them,  with- 
out coming  in  contact,  an  opposite  electrical  condition. 
This  phenomenon  is  called  Electrical  Induction. 

.,    .  This  effect  arises  from  the  general  law  of  electrical  attrac- 

.bxplain      the          .  ° 

phenomena  of  tion  and  repulsion.  A  body  in  its  natural  condition  contains 
induction.  equal  quantities  of  positive  and  negative  electricities,  and  when 

this  is  the  case,  the  two  neutralize  each  other,  and  remain  in  a  state  of  equili- 
brium. But  when  a  body  charged  with  electricity  is  brought  ,into  proximity 
with  a  neutral  body,  disturbance  immediately  ensues.  The  electrified  body, 
by  its  attractive  and  repulsive  influence,  separates  the  two  electricities  of  the 
neutral  body,  repelling  the  one  of  the  same  kind  as  itself,  and  attracting  the 
other,  which  is  unlike,  or  opposite.  Thus,  if  a  body  electrified  positively  be 
brought  near  a  neutral  body,  the  positive  electricity  of  the  neutral  body  will 
be  repelled  to  the  most  remote  part  of  its  surface,  but  the  negative  electricity 
will  be  attracted  to  the  side  which  is  nearest  the  disturbing  body.  Between 
these  two  regions  a  neutral  line  will  separate  those  points  of  the  body  over 
which  the  two  opposite  fluids  are  respectively  distributed. 

pJO  311  Let  CAD,  Fig.  31 1,  be  a  metallic 

cylinder  placed  upon  an  insulating 

0C  support,  with  two  pith  balls  sus- 

s j 1 v  pended  at  one  end,  as  at  D.     If 

(I j         Jr&      now  an    electrified    body,    E,    be 
PI  H        brought  near  to  one  end  of  the  cyl- 

inder, the  balls  at  the  other  ex- 
tremity will  immediately  diverge 
from  one  another,  showing  the  pres- 


ence of  free  electricity.     This  does 
not  arise  from  a  transfer  of  any  'of 

the  electric  fluid  from  E  to  C,  for  upon  withdrawing  the  electrified  body, 
E,  the  balls  will  fall  together,  and  appear  unelectrified  as  before ;  but  the 
electricity  in  E  decomposes  by  its  proximity  the  combination  of  the  two 
electricities  in  the  cylinder,  CAD,  attracting  the  kind  opposite  to  itself 
toward  the  end  nearest  to  it,  and  repelling  the  same  kind  to  the  further 
end.  The  middle  part  of  the  cylinder,  A,  which  intervenes  between  the 
two  extremities,  will  remain  neutral,  and  exhibit  neither  positive  nor  negative 
electricity. 


ELECTRICITY.  377 

FIG.  312.  If  three    cylinders    are 

placed  in  a  row,  touching 

one anotner> as m -^o' si •* i 

and  a  Positively  electnfied 
body,  E,  be  brought  in. 
proximity  to  one  extremity, 
the  electricities  of  the  cyl- 
inders will  be  decomposed, 

the  negative  being  accumulated  in  N",  and  the  positive  repelled  to  P.  If  in 
this  condition  the  cylinder  P  be  first  removed,  and  then  the  electrified  body, 
the  separate  electricities  will  not  be  able  to  unite,  as  in  the  former  experi- 
ment, but  N  will  remain  negatively,  and  P  positively  electrified. 

Explain  the  These  experiments  explain  why  an  electrified 
surface  attracts  a  neutral,  or  unelectrified  body, 
™  un-  SUCD-  as  a  Pita  kail.  I*  is  not  that  electricity 
electrified  body.  causes  attractions  between  excited  and  unex- 
cited  bodies,  the  same  as  between  bodies  oppositely  ex- 
cited ;  but  that  the  pith  ball  is  first  rendered  opposite  by 
induction,  and  attracted  in  consequence  of  this  opposition. 
A  pith  ball  at  a  few  inches  distance  from  an  electrified 
surface,  is  charged  with  electricity  by  induction  ;  and  the 
kind  being  contrary  to  that  of  the  surface,  attraction  en- 
sues ;  when  the  two  touch,  they  become  of  the  same  kind 
by  conduction. 

A  person  may  also  receive  an  electric  shock  by  induction.  Thus,  if  a  per- 
son stand  close  to  a  large  conductor  strongly  charged  with  electricity,  ho 
•will  be  sensible  of  a  shock  when  this  conductor  is  suddenly  discharged. 
This  shock  is  produced  by  the  sudden  recomposition  of  the  fluids  in  the 
body  of  the  person,  decomposed  by  the  previous  inductive  action  of  the 
conductor. 

what  is  an  748.  An  electrical  machine  is  an  apparatus, 
cSl?0*1  ma~  ty  means  of  which  electricity  is  developed  and 

accumulated,  in  a  convenient  manner  for  the 
purposes  of  experiment. 

All   electrical    machines    consist    of   three 

principal  parts,  the  rubber,  the  body  on 
maehina  con-  wnOse  surface  the  electric  fluid  is  evolved, 

and  one  or  more  insulated  conductors,  to 
which  this  electricity  is  transferred,  and  on  which  it  is 
accumulated. 


378 


WELLS'S   NATURAL   PHILOSOPHY. 


Describe   the 
Sr0  ™£i5 


Electrical  machines  are  of  two  kinds,  the 
Plate  and  cylinder  machines.  They  derive 
tlieir  names  from  the  shape  of  the  glass  em- 
ployed to  yield  the  electricity. 

FIG.  313.  The  plate  electrical  machine,  winch  is 

represented  in  Fig.  313,  consists  of  a 
large  circular  plate  of  glass  mounted 
upon  a  metallic  axis,  and  supported  up- 
on pillars  fixed  to  a  secure  base,  so  that 
the  plate  can,  by  means  of  a  handle,  w, 
be  turned  with  ease.  Upon  the  sup- 
ports of  the  glass,  and  fixed  so  as  to 
press  easily  but  uniformly  on  the  plate, 
are  four  rubbers,  marked  r  r  r  r  in  the 
figure  ;  and  flaps  of  silk,  s  s,  oiled  on  one 
side,  are  attached  to  these,  and  secured 
to  fixed  supports  by  several  silk  cords. 
When  the  machine  is  put  in  motion, 
these  flaps  of  silk  are  drawn  tightly 
against  the  glass,  and  thus  the  friction  is 
increased,  and  electricity  excited.  Tho 

points-;?  2?  collect  the  electricity  from  the  glass  as  it  revolves,  and  convey  it  to 

the  prime  conductor,  c,  which  is  insulated  and  supported  by  the  glass  rod,  g. 
The  cylinder  electrical  machine  represented  by 

Fig.  314,  consists  of  a  glass  cylinder,  so  arranged 

that  it  can  be  turned  on  its  axis  by  a  crank,  and 

supported  by  two  uprights  of  wood,  dried  and 

varnished.     F  S  indicates  the  position  and  ar- 

rangement of  the  rubber  and  silk,  and  Y  that 

of  the  prime  conductor.     The  principle  of  the  con- 

struction of  the  cylinder  machine  is,  in  every 

respect,  the  same  as  that  of  the  plate  machine. 

"What    is    the          ^ne  rubber  °f  an  electrical  ma-  . 

construction  of     chine  consists  of  a  cushion  stuffed 

the  rubber  ?  ....  ,  ,        .  , 

with    hair,    and    covered    with 

leather,  or  some  substance  which  readily  generates  electricity  by  friction. 
The  efficiency  of  the  machine  is  greatly  increased  by  covering  the  cushion 
with  an  amalgam,  or  mixture  of  mercury,  tin,  and  zinc.*  "  * 

In  the  ordinary  working  of  the  machine,  the  rubber  is  connected  by  a  chain 
with  the  ground,  from  whence  the  supply  of  electricity  is  derived. 

*  The  best  composition  of  the  amalgam  is  two  parts,  by  weight,  of  zinc,  one  of  tin,  and 
BIX  of  mercury.  The  mercury  is  added  to  the  mixture  of  the  zinc  and  tin  when  in  a  fluid 
state,  and  the  whole  is  then  shaken  in  a  wooden  box  until  it  is  cold  ;  it  is  then  reduced  to 
a  powder,  and  mixed  with  a  sufficient  quantity  of  lard  to  reduce  it  to  the  consistency  of 
paste-  A  thin  coating  of  this  paste  is  spread  over  the  cushion  ;  but  before  this  is  done,  all 
darts  of  the  machine  should  be  carefully  cleaned  and  warmed. 


ELECTRICITY.  379 

•„__       .     •  The  receiver  of  electricity  from  an  electrical  machine  is 

conductor     of     called  the  prime  conductor.     It  usually  consists  of  a  thin  brass 


r>  or  a  brass  rod,  mounted  on  a  glass  pillar,  or  some 
other  insulating  material. 
To  put  the  electrical  machine  in  good  order,  every  part  must  be  dry  and 
clean,  because  dust  or  moisture  would,  by  their  conducting  power,  diffuse  the 
electric  fluid  as  fast  as  accumulated.  As  a  general  rule,  it  is  highly  essential 
that  the  atmosphere  should  be  in  a  dry  state  when  electrical  experiments  are 
made,  as  the  conducting  property  of  moist  air  prevents  the  collection  of  a  su£ 
ficient  amount  of  electricity  for  the  production  of  striking  effects.  In  the 
winter,  the  experiments  succeed  best  when  performed  in  the  vicinity  of  a 
fire;  and  it  is  advisable  to  place  tho  apparatus  in  front  of  the  fire  for  some 
time  before  it  is  employed. 

Electricity  is  developed  by  the  action  of  an  electrical  ma- 
Explain      the  J  J 

method        in     clime  in  essentially  the  same  manner  as  it  is  in  a  simple  glass 

trica?  machine  tube  ^  friction-  When  the  glass  cylinder  or  plate  is  turned 
develops  elec-  round  by  the  handle,  the  friction  between  the  glass  and  the 
tricity?  rubber  excites  electricity;  positive  electricity  being  developed 

upon  the  glass,  and  negative  upon  the  rubber.  When  the  points  of  the  prime 
conductor  are  presented  to  the  revolving  glass  plate  or  cylinder,  the  positive 
electricity  is  immediately  transferred  to  it,  and  it  emits  sparks  to  any  conduct- 
ing substance  brought  near.  The  electricity  thus  abundantly  excited  is  sup- 
plied from  the  earth  to  the  rubber  (by  means  of  a  chain  extending  to  the 
ground),  and  the  rubber  is  continually  having  its  supply  drawn  from  it  by  the 
force  called  into  action  by  friction  with  the  glass.  That  the  electricity  is  de- 
rived from  this  source  is  evident  from  the  fact  that  but  a  small  quantity  of 
electricity  can  be  excited  when  the  metallic  connection  between  the  rubber 
and  the  ground  is  removed.  For  this  reason  the  chain  must  always  be 
attached  to  the  rubber  when  it  is  desired  to  develop  positive  electricity,  and 
to  the  prime  conductor  when  negative  electricity  is  required. 

According  to  the  theory  of  a  single  fluid,  the  excitement  of  electricity  is  as 
follows  :  —  the  friction  of  the  glass  and  silk,  by  disturbing  the  electrical  equi- 
librium deprives  the  rubber  of  its  natural  quantity  of  electricity,  and  it  is 
therefore  left  in  a  negative  state,  unless  a  fresh  quantity  be  continually  drawn 
from  the  earth  to  supply  its  place.  The  surplus  quantity  is  collected  on  the 
prime  conductor,  which  thereby  becomes  charged  with  positive  electricity. 
On  the  hypothesis  of  two  electric  fluids,  the  same  frictional  action  causes 
the  separation  of  the  vitreous  from  the  resinous  electricity  in  the  rubber,  which 
therefore  remains  resinously  charged,  unless  there  be  a  connection  with  the 
earth  to  restore  the  proportion  of  vitreous  electricity  of  which  the  rubber  has 
been  deprived. 

Various  other  arrangements  have  been  devised  for  the  pro- 

boner'1  be  "used     duction  and  accumulation  of  electricity.     High-pressure  steam 

as  un  electrical     escaping  from  a  steam-boiler  carries  with  it  minute  particles 

of  water,  and  the  friction  of  these  against  the  surface  of  tho 

jet  from  which  the  steam  issues  produces  electricity  in  great  abundance.     A 


380 


WELLS'S   NATURAL   PHILOSOPHY. 


steam-boiler,  properly  arranged  and  insulated,  therefore  constitutes  a  most 
powerful  electrical  machine  ;  and  by  means  of  an  apparatus  of  this  character, 
constructed  some  time  since  in  London,  flashes  of  electricity  were  caused  to 
emanate  from  the  prime  conductor  more  than  22  inches  in  length. 

749.  The  Insulating  Stool,  which  is  a  usual  J,JQ  3  g 
iuiating  s"oo°?  appendage  to  an  electrical  machine,  consists  of 
a  board  of  hard-baked  wood,  supported  on 
glass  legs  covered  with  varnish.  (See  Fig.  315.)  It  is  useful  for 
insulating  any  body  charged  with  electricity  ;  and  a  person 
standing  upon  such  a  stool,  and  in  communication  with  a 
prime  conductor,  will  become  charged  with  electricity. 

Discharging    Rods    are    brass  FIG.  316. 

What  are  Dis-  ,  .  ,     .     „ 

charging  Rods?     rods  terminating  with  balls,   or 

with  points,  fixed  to  glass  handles. 

With  these  rods  electricity  may  be  taken  from  a 

conductor  without  allowing  the  electrical  charge 

to  pass  through  the  body  of  the  operator.     Their 

construction  is  represented  in  Fig.  316. 

An  instrument  called  the  "Universal 

.  317.  Discharger,"    used    to   convey   strong 

charges  of  electricity  through  various 
substances,  is  represented  by  Fig.  317. 
It  consists  of  two  glass  standards, 
through  the  top  of  which  two  metallic 
wires  slide  freely;  these  wires  are 
pointed  at  the  end,  t,  but  have  balls 

screwed  upon  them  ;  the  other  ends  are  furnished  with  rings.     The  balls  rest 

on  a  table  of  boxwood,  into  which  a  slip  of  ivory,  or  thick  glass,  is  inlaid. 

Sometimes  a  press,  p',  is  substituted  for  the  table,  between  which  any  sub- 

stance necessary  to  be  pressed,  during  the  discharge,  is  held  firm. 

750.  An  Electrophorus  is  a  simple  appara- 
j^  jn  w^ich  a  small  charge  of  electricity  may- 
be generated  by  induction  ;  and  this,  communicated  suc- 
cessively to  an  insulated  conductor,  may  produce  a  charge 
of  indefinite  amount. 

It  consists  of  a  circular  cake  of  resin  (shell-lac),  r,  Fig.  318, 
action  of  the  laid  upon  a  metallic  plate  ;  upon  this  cake,  the  surface  of  which 
electrophorus.  has  been  negatively  electrified  by  rubbing  it  with  dry  silk  or  fur, 
is  placed  a  metallic  cover,  M,  somewhat  smaller  in  diam- 
eter, and  furnished  with  a  glass  insulating  handle,  h. 
The  negative  electricity  of  the  resin,  by  acting  induc- 
tively upon  the  two  electricities  combined  in  the  cover, 
separates  them  —  the  positive  being  attracted  to  the 
under  surface,  and  the  negative  repelled  to  the  upper, 
on  touching  the  cover  with  the  finger,  all  the  negative 


what    is  an 


FIG.  318. 


ELECTRICITY. 


381 


electricity  will  escape,  and  the  positive  electricity  alone  remains,  which  is 
combii.ed  with  the  negative  electricity  of  tha  cake  of  resin,  so  long  as  the 
cover  is  in  contact  with  it.  If  we  now  remove  the  cover  by  its  insulating 
handle,  the  positive  electricity,  which  was  before  held  at  the  lower  part  of 
the  cover  by  the  inductive  action  of  the  resin,  will  become  free,  and  may  be 
imparted  to  any  insulated  conductor  adapted  to  receive  it.  The  same  pro- 
cess may  be  repeated  indefinitely,  as  the  resinous  cake  loses  none  of  its  elec- 
tricity, but  simply  acts  by  induction,  and  thus  an  insulated  conductor  may  be 
charged  to  any  extent.  J 

751.  /An  Electroscope  is  an  instrument  em- 
ployed to  indicate  the  presence  of  free  elec- 


What    is     an 
Electroscope  ? 


tricity. 


what  is  the 


^  usually  consists  of  two  light  conducting 
bodies  freely  suspended,  which  in  their  natural 
state  hang  vertically  and  in  contact.  When 
electricity  is  imparted  to  them,  they  repel  each  other,  and 
the  amount  of  their  divergence  is  proportioned  to  the 
quantity  of  electricity  diffused  on  them. 

The  simplest  form  of  the  electroscope,  called  the  -"pith-ball  electroscope," 
consists  of  two  pith-balls  suspended  by  silk  threads.  When  an  excited  body 
is  presented,  the  balls  will  be  first  attracted,  but  immediately  acquiring  the 
same  degree  of  electricity  as  the  exciting  body,  they  repel  each  other.  An- 
other form  of  the  pith-ball  electroscope,  represented  at  B,  Fig.  319,  consists  of 
two  pith-balls  suspended  by  conducting  threads  within  a  glass  jar,  and  con- 
nected with  the  brass  cap,  m.  On  touching  the  brass  cap  with  an  electrified 
FiG.  319.  body;  the  two  balls  being  similarly  electri- 

fiod,  will  repel  each  other.  C,  Fig.  319, 
represents  a  more  delicate  electroscope; 
two  slips  of  gold  leaf,  g  g',  being  substituted 
for  the  pith-balls.  If  an  excited  substance, 
c,  be  brought  near  the  cap  of  brass,  the 
leaves  will  instantly  diverge.  The  best 
electrometers  are  carefully  insulated,  so  that 
the  electricity  communicated  to  the  balls  or 
leaves  may  not  be  too  soon  dissipated. 

Electroscopes  merely  indicate 
the  presence  of  an  electrically  excited  body  :  they  do  not 
measure  the  quantity,  either  relatively  or  absolutely,  of 
the  electricity  in  action. 
what   is   an        752.  An  Electrometer  is  an  instrument  for 

trometer  ?       measurjng  faQ  quantity  Of  electricity. 

The  most  simple  form  of  the  electrometer  is  represented  at  A,  Fig.  319.    It 


382 


WELLS'S   NATURAL   PHILOSOPHY. 


the        Torsion 
Balance. 


FIG.  320. 


consists  of  a  semicircle  of  varnished  paper,  or  ivory,  fixed  upon  a  vertical 
rod.  -From  the  center  of  the  semicircle  a  light  pith-ball  is  suspended,  and 
the  number  of  degrees  through  which  the  ball  is  attracted  or  repelled  by  any 
body  brought  in  proximity  to  it,  indicates  in  a  degree  the  active  quantity  of 
electricity  present.  No  very  accurate  results,  however,  can  be  obtained  with 
this  apparatus ;  and  for  accurate  investigation,  instruments  of  more  ingenious 
and  complicated  construction  are  used. 

The  electrometer  usually  employed  for  measuring  with 
great  accuracy  small  quantities  of  electricity,  is  that  of 
Coulomb's,  usually  called  the  Torsion  Balance. 

Ex  lain  the  The  construction  of  tnis  instrument  is  as  follows : — A  needle, 
construction  of  or  stick  of  shell-lac,  bearing  upon  one  end  a  gilded  pith-ball,  is 
suspended  by  a  fiber  of  silk  within  a  glass  vessel — the  needle 
being  so  balanced,  that  it  is  free  to  turn  horizontally  around 
the  point  of  suspension  in  every  direction.  When  the  pith-ball  is  electrified 
by  induction,  the  repellent  force  causes  the  needle  to  turn  round,  and  this 
produces  a  degree  of  torsion,  or  twist  in  the  fiber  which  suspends  it ;  and  the 
tendency  of  the  fiber  to  untwist,  or  return  to  its  original  position,  measures 
the  force  which  turns  the  needle. 
Within  the  glass  vessel,  which  is  cylin- 
drical, a  graduated  circle  is  placed, 
which  measures  the  angle  through 
which  the  needle  is  deflected.  In  the 
cover  of  the  vessel  an  aperture  is  made, 
through  which  the  electrified  body  may 
be  introduced,  whose  force  it  is  desired 
to  indicate  and  measure  by  the  ap- 
paratus. Fig.  320  represents  the  con- 
struction and  appearance  of  the  torsion 
balance. 

By  means  of  the 

torsion     balance, 

Coulomb    proved 

that  the  law  of 
electrical  attraction  and  re- 
pulsion, as  influenced  by  dis- 
tance, is  the  same  as  the  law 
of  gravitation  ;  that  is,  the  force  varies  inversely  as  the 
square  of  the  distance. 

what  is  a  Ley-        ^53.  The  Leyden  Jar  is  a  glass  vessel  used 
den  jar?        for  foe  purpose  of  accumulating  electricity  de- 
rived from  electrically  excited  surfaces. 


What  import- 
ant law  of 
electricity  has 
been  proved  by 
the  torsioa  bal- 
ance? 


ELECTRICITY.  383 

The  principle  of  the  Leyden  Jar  may  j<IG  321 

be  best  explained  by  describing  what  is 
Btriictio-iofthe  called  the  "coated,"  or  "fulminating 

pane."  This  consists  of  a  glass  plate,  Fig. 
321,  a,  having  a  square  leaf  of  tin-foil,  &,  attached  to  each 
side.  If  the  plate  be  laid  upon  a  table,  and  a  chain  from 
the  prime  conductor  of  an  electrical  machine  be  brought 
in  contact  with  the  tin-foil  upon  one  side,  the  plate  will 
become  charged— the  upper  side  with  positive,  and  the 
under  with  negative  electricity. 

If  two  such  conductors,  as  the  plates  of  tin-foil  attached  to 
Sated  ma3pane  a  pane  of  glass,  be  strongly  charged  with  electricity  in  the 
produce  anelec-  manner  described,  and  then,  by  means  of  the  human  body,  be 

put  in  communication — which  may  be  done  by  touching  one 
plate  with  the  fingers  of  one  hand,  and  the  other  with  the  fingers  of  the  other 
hand — the  two  electric  fluids  in  rushing  together,  pass  through  the  body,  and 
produce  the  phenomenon  known  as  the  electric  shock. 

754.  The  Leyden  Jar  is  constructed  upon  the  same  prinei- 

In   what  man-    ^Q  ag  ^e  coated  pane,  and  its  discovery,  accompanied  with 

rinciple  of  the    the  first  experience  of  the  nervous  commotion  known  as  the 

electric  shock,  occurred  in  this  way:    In  1746,  while  some 

scientific  gentlemen  at  Leyden,  in  Holland,  were  amusing  them- 
selves with  electrical  experiments,  it  occurred  to  one  of  them  to  charge  a 
tumbler  of  water  with  electricity,  and  learn  by  experiment  whether  it  would 
affect  the  taste.  Accordingly,  having  fixed  a  metallic  rod  in  the  cork  of  a 
bottle  filled  with  water,  he  presented  it  to  the  electrical  machine  for  the  pur-» 
pose  of  electrifying  the  water,  holding  at  the  same  time  the  bottle  in  his  hand 
by  its  external  surface,  without  touching  the  metallic  rod  by  which  the  elec- 
tricity was  conducted  to  the  water.  The  water,  which  is  a  conductor,  re" 
ceived  and  retained  the  electricity,  since  the  glass,  a  non-conductor,  by  which 
it  was  surrounded,  prevented  its  escape.  The  presence  of  free  electricity  in 
the  water,  however,  induced  an  opposite  electricity  on  the  outside  of  the  glass, 
and  when  the  operator  attempted  to  remove  the  rod  out  of  the  bottle,  he 
brought  the  two  electricities  into  communication  by  means  of  his  hand,  and 
received,  for  the  first  time,  a  severe  electric  shock.  Nothing  could  exceed 
the  astonishment  and  consternation  of  the  operator  at  this  unexpected  sensa- 
tion, and  in  describing  it  in  a  letter  immediately  afterward  to  the  French 
philosopher  Reaumur,  he  declared  that  for  the  whole  kiLgdom  of  France  he 
would  not  repeat  the  experiment.  • 

The  experiment,  however,  was  soon  repeated  in  different  parts  of  Europe, 
and  the  apparatus  by  which  it  was  prgduced  received  a  more  convenient 
form,  the  water  being  replaced  by  some  better  conducting  substances,  as 
metal  filings,  for  which  tin-foil  was  afterward  substituted. 

The  Leyden  Jar,  as  usually  constructed,  con- 

Describe      the        «    ,          /»  i  •  -rv        onr»     i  •  i 

construction  of    sists  of  a  glass  jar,  Fig.  322,  having  a  wide 
the  Leyden  jar.    mout^  an£  coated,  externally  and  internally,  to 


384 


WELLS'S   NATURAL   PHILOSOPHY. 


FIG.  322. 


within  two  or  three  inches  of  the  mouth, 
or  to  the  line  a  b,  with  tin-foil.  A  wooden 
cover,  well  varnished,  is  fitted  into  the 
mouth  of  the  jar,  through  which  a  stout 
brass  wire,  furnished  with  a  ball,  passes, 
having  a  chain  or  wire  attached  to  its 
lower  end,  so  as  to  be  in  contact  with 
the  inside  coating. 

HOW  is  a  Le  ^  Leyden  jar  is  charged 
den  jar  charg-  by  presenting  the  brass  ball 

at  the  end  of  the  rod  of  the 
jar  to  a  prime  conductor  of  an  electrical  machine  in 
action,  or  to  any  other  excited  surface.  To  charge  a  jar 
strongly,  it  is  necessary  that  the  outside  coating  should  be 
directly  or  indirectly  connected  with  the  ground. 
HOW  is  a  Ley.  A  Leyden  jar  is  discharged  by  effecting  a 
charged  ***"  communication  between  the  outer  and  inner 

surfaces  by  means  of  a  good  conductor. 

If,  when  we  have  charged  the  jar,  we  hold  the  exterior  coating  in  ono 
hand  and  touch  the  knob  with  the  other,  a  spark  is  observed,  and  the  peculiar 
sensation  of  the  electric  shock  experienced. 

Any  number  of  persons  can  receive  a  shock  at  the  same  time  by  forming  a 
chain  by  holding  each  other's  hands — the  first  person  in  the  circle  touching 
the  external  coating  of  the  jar,  and  the  last  the  knob. 

Where  does  When  a  Leyden  jar  is  charged,  the  electricity  resides  wholly 
the  electricity  on  the  surface  of  the  glass;  the  metallic  coatings  having  no 
restdeTden^ar  other  effect  than  to  conduct  the  electricity  to  the  surface  of 
the  glass,  and,  when  there,  afford  it  a  free  passage  from  point 
to  point. 

The  power  of  a  Leyden  jar  will  therefore  depend  upon  its  size,  or  extent 
of  surface. 

As  very  large  jars  are  inconvenient  and 
expensive,  very  strong  charges  of  electricity 
are  obtained  by  combining  a  number  of  jars 
together. 

A  combination   of 

What     is     an 

Electrical  Bat.    Leyden  jars,  so   ar- 
ranged that  they  may 
be  all   charged   and  discharged 
together,  constitutes  an  Electri- 


Fia.  S23. 


ELECTRICITY. 


385 


What  experi- 
ments illustrate 
the  attractive 
and  repulsive 
forces  of  elec- 
tricity ? 


FIG.  324. 


cai  Battery.  This  may  be  effected  by  forming  a  connec- 
tion between  all  the  wires  proceeding  from  the  interiors 
of  the  jars,  and  also  connecting  all  their  exterior  coatings. 

Such  an  arrangement  is  represented  by  Fig.  323.  The  discharge  of  elec- 
tricity from  such  a  combination  is  accompanied  by  a  loud  report;  and  when 
the  number  of  the  jars  is  considerable,  animals  may  be  killed,  metal  wires 
be  melted,  and  other  effects  produced  analogous  to  those  of  lightning. 

755.  By  means  of  an  electrical  machine  and  the  Ley  den 
Jar,  many  interesting  and  amusing  electrical  experiments 
may  be  performed. 

The  phenomenon  of  the  repulsion  of  substances  similarly 
electrified,  may  be  illustrated  by  means  of  a  doll's  head  cov- 
ered witli  long  hair.  When  this  is  at- 
tached to  the  prime  conductor  of  an  elec- 
trical machine,  the  hairs  stand  erect,  and 
give  to  the  head  a  most  exaggerated  ap- 
pearance of  fright.  See  Fig.  324. 

The  same  thing  may  be  shown  by  plac- 
ing a  person  on  a  stool  with  glass  legs, 
so  that  he  be  perfectly  insulated,  and 
making  him  hold  in  his  hand  a  brass  rod, 
the  other  end  of  which  touches  the  prime 
conductor ;  then  on  turning  the  machine, 
the  hairs  of  the  head  will  diverge  m  all 
directions. 

If  a  small  number  of  figures  are  cut 
out  in  paper,  or  carved  out  of  pith,  and 
an  excited  glass  tube  be  held  a  few- 
inches  above  them  on  a  table,  the  figures 

will  immediately  commence  dancing  up  and  down,  assuming  a  variety  of  droll 
positions.  The  experiment  can  be  shown  better  by  means 
of  an  electrical  machine  than  with  the  excited  tube,  by 
suspending  horizontally  from  the  prime  conductor  a  metal 
disc  a  few  inches  above  a  flat  metal  surface  connected  with 
the  earth,  on  which  the  figures  are  placed.  On  working 
the  machine,  the  figures  will  dance  in  a  most  amusing 
m  inner,  being  alternately  attracted  and  repelled  by  each 
plate.  See  Fig.  325. 

The  electrical  bells,  Fig.  326,  which  are 
What    is    the  ,  . 

experiment  of     rung  by  electric  attraction  and  repulsion, 

bells  ?electrical  are  good  illustrations  of  these  forces.  Where 
three  bells  are  employed,  the  two  outer 
bells  A  and  B,  are  suspended  by  chains,  but  the  central 
one  and  the  two  clappers  hang  from  silken  strings.  The 
middle  bell  is  connected  with  the  earth  by  a  chain  or  wire. 

17 


FIG.  323. 


X 


386 


WELLS'S   NATURAL   PHILOSOPHY. 


Upon  working  the  machine,  the  outer  bells  become  positively  electrified,  and 
FIG  326  *ne  m*ddle  OIie?  which  is  insulated  from  tho 

prime  conductor,  becomes  negative  by  in- 
duction. The  little  clappers  between  them 
are  alternately  attracted  and  repelled  by  tho 
outer  and  inner  bells,  producing  a  constant 
ringing  as  long  as  tho  machine  is  in  action. 
It  was  by  attaching  a  set  of  bells  of  this 
kind  to  his  lightning-conductor,  that  Dr. 
Franklin  received  notice,  by  their  ringing, 
of  the  passage  of  a  thunder-cbud  over  his 
apparatus. 

Let  a  skein  of  linen  thread  be  tied  in  a 
knot  at  each  end,  and  let  one  end  of  it  be  attached  to  some  part  of  tho  con- 
ductor of  a  machine.  "When  the  machine  is  worked  the  threads  will  become 
electrified,  and  will  repel  each  other,  so  that  the  skein  will  swell  out  into  a 
form  resembling  the  meridians  drawn  upon  a  globe. 

If  we  ignite  the  extremity  of  a  stick  of  sealing-wax,  and  bring  .the  melted 
wax  near  to  the  prime  conductor  of  a  machine,  numerous  line  filaments  of 
wax  will  fly  to  the  conductor,  and  will  adhere  to  it,  forming  upon  it  a.  sort 
of  network  like  wool.  This  is  a  simple  case  of  electrical  attraction.  The 
experiment  will  succeed  best  if  a  small  piece  of  wax  is  attached  to  the  end 
of  a  metal  rod.  •? 

756.  When  a  current  of  electricity  passes 
through  a  good  conductor  of  sufficient  size  to 
carry  off  the  whole  quantity  of  electricity 
easily,  the  conductor  is  not  apparently  affected  by  its 
passage  ;  but  if  the  conductor  is  too  small,  or  too  imper- 
fect to  transmit  the  electric  fluid  readily,  very  striking 
effects  are  produced — -the  conductor  being  not  unfre- 
quently  shivered  to  pieces  in  an  instant. 

The  mechanical  effects  exerted  by  electricity  in  passing 
through  imperfect  conductors,  may  be  illustrated  by  many 
simple  experiments. 

If  we  transmit  a  strong  charge  of  electricity  through  wator, 
:   the  liquid  will  be  scattered  in  every  direction. 

A  rod  of  wood  half  an  inch  thick  may  be  split  by  a  strong  charge  from  a 
Leyden  jar,  or  battery,  transmitted  in  the  direction  of  its  fibers. 

If  we  place  a  piece  of  dry  writing-paper  upon  the  stand  of  a  universal  dis- 
charger, and  then  transmit  a  charge  through  it,  the  electricity,  if  sufficiently 
strong,  will  rupture  the  paper. 

If  we  hold  the  flame  of  a  candle  to  a  metallic  point  projecting  from  the 
prime  conductor  of  an  electrical  machine  in  action,  tho  current  of  air  caused 


What  effect  has 


What  experi- 
ments illustrate 
the  mechanical 
effects  of  elec- 
tricity? 


ELECTRICITY.  387 


by  the  issuing  of  a  current  of  electricity  from  the  point,  will  bo  sufficient  to 
deflect  the  flame,  and  even  blow  it  out. 

HOW  does  dec.        757.  The  passage  of  electricity  from    one 

heat?  ev°lve    substance  to  another  is  generally  attended  with 

an  evolution  of  heat,  and  a  current  of  electricity 

passing  over  an  imperfect  conductor,  raises  its  temperature. 

The  temperature  of  a  good  conductor  of  sufficient  size 

to  allow  the  electric  fluid  to  pass  freely,  is  not  affected  by 

the  transmission  of  a  current  of  electricity  ;  but  if  its  size 

is  disproportionate  to  the  quantity  of  fluid  passing  over 

it,  it  will  be  heated  to  a  greater  or  less  degree. 

If  a  small  charge  of  electricity  be  passed  through  small  metal  wire  a  few 
inches  in  length,  its  temperature  will  be  sensibly  elevated ;  if  the  charge  be 
increased,  the  wire  may  be  made  red  hot,  and  even  melted  and  vaporized. 

The  worst  conductors  of  electricity  suffer  much  greater  changes  of  tem- 
perature by  the  same  charge  than  the  best  conductors.  The  charge  of  elec? 
tricity  which  only  elevates  the  temperature  of  one  conductor,  will  sometimes 
render  another  red  hot,  and  will  volatilize  a  third. 

The  heat  developed  in  the  passage  of  electricity  through 
combustible  or  explosive  substances,  which  are  imperfect 
conductors,  causes  their  combustion  or  explosion. 

If  gunpowder  be  scattered  over  dry  cotton  loosely  wrapped  round  one  end 
of  a  discharging-rod,  it  may  be  ignited  by  the  discharge  of  a  Leyden  jar. 

In  the  same  way  powdered  resin  may  be  inflamed. 

Ether  or  alcohol  may  be  also  fired  by  passing  through  it  an  electric  dis- 
charge. Let  cold  water  be  poured  iuto  a  wine  glass,  and  let  a  thin  stratum 
of  ether  be  carefully  poured  upon  it.  The  ether  being  lighter  will  float  on 
the  water.  Let  a  wire  or  chain  connected  with  the  prime  conductor  of  a 
machine  be  immersed  in  the  water,  and,  while  the  machine  is  in  action,  pre- 
sent a  metallic  ball  to  the  surface  of  the  ether.  The  electric  charge  will  pass 
from  the  water  through  the  ether  to  the  ball,  and  will  ignite  the  ether. 

If  a  person  standing  on  an  insulated  stool  touches  the  prime  conductor 
with  one  hand,  and  with  the  other  transmits  a  spark  to  the  orifice  of  a  gas- 
pipe  from  which  a  current  of  gas  is  escaping,  the  gas  will  be  ignited. 

By  the  friction  of  the  feet  upon  a  dry  woolen  carpet,  sufficient  electricity 
may  be  often  excited  in  the  human  body  to  transmit  a  spark  to  a  gas-burner, 
and  thus  ignite  the  gas. 

If  we  bring  a  candle  with  a  long  snuff,  that  has  just  boon  extinguished, 
near  to  a  prime  conductor,  so  that  the  spark  passes  from  the  conductor, 
through  the  smoke,  to  the  candle,  it  may  be  relighted. 

is  the  electric        The  electric  fluid  is  not  itself  luminous  ;  but 
its  motion  over  imperfect  conductors,  or  from 


388  WELLS'S   NATURAL   PHILOSOPHY. 

one  conducting  substance  to  another,  is  generally  attended 
with  an  exhibition  of  liht. 


t  r.,i  t  h  ^e  stronSest;   electric   charges   that   can  be  accumulated 

regarded  as  a  in  a  body  will  never  aftbrd  the  least  appearance  of  light  .so 
Pl™Ptrlcrt  »  °f  *on=>  as  a  s^ato  °?  elec'tric  equilibrium  exists,  and  the  electric 
fluids  are  at  rest.  Light,  therefore,  must  not  be  regarded  as 
a  property  of  electricity,  but  as  the  result  of  a  disturbance  occasioned  by 
electricity. 

Th.9  fur  of  a  cat  sparkles  when  rubbed  with  the  hand  in 
fur  ^of  a  cat  cold  weather.  The  reason  of  this  is,  that  the  friction  between 
sparkle?  ^e  hand  and  the  far  produces  an  excitation  of  negative  elec- 

tricity in  the  hand,  and   positive  in  the  fur,  and  an  -interchange  of  the  two 
is  accompanied  with  a  spark,  or  appearance  of  light. 
What   i.   the        Whenthefinger,  ^  32t 

form    of    the      or  a  brass  ball  at 
electric  Bpark  ?       th(J  end  of  ft  rod>  ig 

presented  to  the  prime  conductor 

of  an  electrical  machino  in  action, 

a  spark  is  produced  by  the  passage 

of  the  fluid  from  the  conductor  to 

the  finger  or  the  metal.      This 

spark  has  an  irregular  zigzag  form,  resembling,  more  or  less,  the  appearance 

of  lightning,  as  shown  in  Fig.  327. 

upon  what  doe.  The  lenSth  of  the  electric  spark  will  vary 
Sr??th£>ark  with  lne  power  of  the  machine.  A  very 
depend?  powerful  machine  will  so  charge  its  prime 

conductor,  that  sparks  may  be  taken  from  it  at  the 
distance  of  30  inches. 

If  the  part  of  ei-  FlG>  328> 

How    does    a  -  "' 

point  influence       ther  of  the  clectn- 

Cally  excited  b°d" 
ies  which  is   pre- 

eented  to  the  other  has  tho  form 

of  a  point,  the  electric  fluid  will 

escape,  not  in  the  form  of  a  spark, 

but  as  a  brush,  or  pencil  of  light, 

the  diverging  rays  of  which  have  sometimes  a  length  of  two  or  three  inched. 

Fig.  328  represents  this  appearance. 

A  substance  parting  with  electricity  generally  exhibits  an  irregular  spark, 
or  flash  of  light;  while  a  substance  absorbing  electricity  exhibits  a  brush  or 
glow  of  light. 

what  ia  the        The  rapidity  of  the  electric  light  is  marvel- 
ous  :  and  it  has  been  experimentally  shown 


ELECTRICITY. 


389 


that  the    duration  of  the   light  of  the  spark  does    not 
exceed  the  one-millionth  part  of  a  second.* 

When  the  continuity  of  a  substance  conducting  electricity  is  interrupted,  a 
spark  will  be  produced  at  every  point  where  the  course  of  the  conductor  is 
broken. 

A  great  variety  of  beautiful  experiments  may  bo  performed  to  illustrate 
this  principle.  Thus,  upon  a  piece  of  glass  may  be  placed  at  a  short  distance 

from  each  other  any  number  of  bits  or 
FIG.  329.  pieces  of  tin-foil,  as  is  represented  by 

FJS-  329'5  when  the  metal  at  either  end 
is  connected  with  the  prime  conductor 

of  an  electrical  machine,  the  sparks  will 
pass  from  one  piece  of  tin-foil  to  the 
other,  and  form  a  stream  of  beautiful 
light.  By  varying  the  position  of  the 
pieces  of  tin-foil,  letters,  or  any  other  devices  may  be  exhibited  at  the  pleasure 
of  the  operator. 

In  a  like  manner,  by  fasten- 

FIG.  330  ing  by  means  of  lac-varnish  a 

spiral  line  of  pieces  of  tin-foil 

uPon  tne  interior  of  a  tube»  a3 

is  ^presented  in  Fig.  330,  a 
serpentine  line  of  fire  may  be 


Cn  s 

\JV 


made  to  paiss  from  one  end  of  the  tube  to  the  other. 

•  The  arrangement  by  which  this  fact  was  demonstrated  by  Mr.  Wheatstone  of  England, 
may  be  described  as  follows  :—  Considerable  lengths  of  copper  wire  (about  half  a  mile 
being  employed),  are  so  arranged,  that  three  small  breaks  occur  in  its  continuity  —  one  near 
the  outer  coating  of  a  Leyden  jar,  one  near  the  connection  with  the  inner  coating,  and 
another  exactly  in  the  middle  of  the  wire—  so  that  three  sparks  are  seen  at  every  dis- 
charge, one  at  the  break  near  the  source  of  excitation,  another  in  the  middle  of  its  path, 
and  the  third  close  to  the  point  of  returning  connection  ;  these,  by  bending  the  wire,  are 
brought  close  together.  Exactly  opposite  to  this  was  placed  a  metallic  speculum,  fixed 
on  an  axis,  and  made  to  revolve  parallel  to  the  line  of  the  three  sparks.  When  a  spark 
of  light  is  viewed  in  a  rapidly  revolving  mirror,  a  long  line  is  Been  instead  of  a  point.  It 
will  be  obvious  that  three  lines  of  light  will  be  seen  in  the  revolving  mirror  every  time  a 
discharge  takes  place,  and  that  if  the  first  or  the  last  differ  in  the  smallest  portion  of  time, 
these  lines  must  begin  at  different  points  on  the  speculum. 

When  the  mirror  revolved  slowly,  the  position  of  the  lines  was  uniform,  thus  i~ 

but  when  the  velocity  was  increased,  they  appeared  thus  mi.-—  ;  those  pro- 

duced by  the  sparks  at  either  end  of  the  wire  being  constantly  coincident,  but  the  spark 
evolved  at  the  break  in  the  middle  being  slightly  behind  the  other  two.  From  this,  it 
appears  that  the  disturbance  commences  simultaneously  at  either  end  of  a  circuit,  and 
travels  toward  the  middle.  This  has  been  adduced  in  proof  of  the  two  electricities.  It 
was  thus  determined  that  electricity  moves  through  copper  wire  at  a  rate  beyond  288,000 
miles  in  a  second.  It  will  be  evident  to  any  one  considering  the  subject,  that  the  length 
of  the  line  seen  in  the  speculum  depends  on  the  duration  of  the  spark.  Wh6n  the  mirror 
was  made  to  revolve  800  times  in  a  second,  the  image  of  the  spark,  at  10  feet  distance, 
appeared  to  the  eye  of  the  observer  to  make  an  arc  of  about  half  a  degree,  and  from  thii 
its  duration  was  calculated.—  Hunt. 


390  WELLS'S   NATURAL   PHILOSOPHY. 

upon  what  does        758.  The  intensity  of  the  electric  light  de- 

tje  intensity^     pends  j^  upOQ    ^    dengity  Qf    faQ    aCCUlDU- 


ught  depend?  iated  electricity,  and  the  density  and  nature  of 
the  aerial  medium  through  which  the  spark  passes. 

Thus,  the  electric  light,  in  condensed  air,  is  very  bright,  and  in  a  rarefied 
atmosphere  it  is  faint  and  diffusive,  like  the  light  of  the  aurora  borealis  ;  in 
carbonic  acid  gas  the  light  is  white  and  intense  ;  it  is  red  and  faint  in  hydro- 
gen, yellow  in  steam,  and  green  in  ether  or  alcohol. 

How  ma    th  ^'  ^v  means  of  an  air-pump,  the  air  is  exhausted  from  a 

auroral  light  be  long  cylindrical  tube  closed  at  each  end  with  a  metallic  cap, 
imitated  ?  an(^  %  curren^  of  electricity  passed  through  it,  an  imitation  of 

the  appearance  of  the  aurora  borealis  is  produced.  When  the  exhaustion  of 
the  tube  is  nearly  perfect,  the  whole  length  of  the  tube  will  exhibit  a  violet 
red  light.  If  a  small  quantity  of  air  be  admitted,  luminous  flashes  will  be  seen 
to  issue  from  points  attached  to  the  caps.  As  more  and  more  air  is  admitted, 
the  flashes  of  light  which  glide  in  a  serpentine  form  down  the  interior  of  the 
tube  will  become  more  thin  and  white,  until  at  last  the  electricity  will  cease 
to  be  diffused  through  the  column  of  air,  and  will  appear  as  a  glimmering 
light  at  the  two  points. 

759.  The  crackling  noise,  or  sound  which  is  produced 
by  the  electric  discharge,  is  attributed  to  the  sudden  dis- 
placement  of  the   particles    of   air,    or    other   medium 
through  which  the  electric  fluid  passes. 

760.  The  electric  shock,  or  convulsive  sensation  occa- 
sioned by  the  passage  of  the  electric  fluid  through  the 
body  of  a  man,  or  animal,  is  supposed  to  arise  from  a 
momentary  derangement  of  the  organs  of  the  body,  ow- 
ing to  an  imperfection,  or  difference  in  the  conducting 
power  of  the  solids  and  fluids  which  compose  them. 

If  this  derangement  does  not  exceed  the  power  of  the  parts  to  recover  their 
position  and  organization,  a  convulsive  sensation  is  felt,  the  violence  of  which 
is  greater  or  less  according  to  the  force  of  electricity  and  the  consequent  de- 
rangement of  the  organs  ;  but  if  it  exceeds  this  limit,  a  permanent  injury,  or  even 
death,  may  ensue. 

what  are  the  761.  In  the  processes  hitherto  described 
agents  in°nl!  electricity  has  been  developed  by  friction.  In 
eiecti?cltxy?ting  nature  the  agents  which  are  undoubtedly  the 
most  active  in  producing  and  exciting  elec- 
tricity, are  the  light  and  heat  of  the  sun's  rays. 

The  change  of  form  or  state  in  bodies  is  also  one  of  the 
most  powerful  methods  of  exciting  electricity. 


ATMOSPHERIC   ELECTRICITY.  391 

Water,  in  passing  into  steam  by  artificial  heat,  or  in  evaporating  by  the  ac- 
tion of  the  sun  or  wind,  generates -large  quantities  of  electricity.  Thu  crystal- 
lization of  solids  from  liquids,  all  changes  of  temperature,  the  growth  and  de- 
cay oi  vegetables,  are  also  instrumental  m  producing  electrical  phenomena. 

Does  vital  and  Recent  investigations  have  shown  that  vital 
Son  cSite  eiTc"-  action  an(l  all  muscular  movements  in  man 
tricity?  an(j  animals,  develop  or  produce  electricity;  it 

may  also  be  shown  by  direct  experiment  that  a  person 
can  not  even  contract  the  muscles  of  the  arm  without  ex- 
citing an  electrical  action. 

Certain  animals  are  gifted  with  the  extraordinary  power  of  producing  at 
pleasure  considerable  quantities  of  electricity  in  their  system,  and  of  commu- 
nicating it  to  other  animals,  or  substances.  Among  these  the  electrical  eel 
and  the  torpedo  are  most  remarkable,  the  former  of  which  can  send  out  a 
charge  sufficient  to  knock  down  and  stun  a  man,  or  a  horse.  The  electricity 
generated  by  these  animals  appears  to  be  the  same  in  character  as  that  pro- 
duced by  the  electdcal  machine. 

762.  It  has  of  late  become  the  habit  with  many  to  regard 
reason  in  as-  electricity  as  the  agent  of  all  phenomena  in  the  natural  world, 
cribing  mi-  ^ne  cause  of  which  may  not  be  apparent.  For  this  there  is  no 
n!e0™"toP  elec-  good  reason.  Electricity  is  diffused  through  all  matter,  and 
tncity?  jg  ever  active,  and  many  of  its  phenomena  can  not  be  satisfac- 

torily explained ;  but  it  is  governed,  like  all  other  forces  of  nature,  by  cer- 
tain fixed  laws,  and  it  is  by  no  means  a  necessary  agent  in  all  the  operations 
of  nature.  It  therefore  argues  great  ignorance  to  refer  without  examination 
every  mysterious  phenomenon  to  the  influence  of  electricity. 

SECTION    I. 

ATMOSPHERIC     ELECTRICITY. 

Does  electricity       763.  Electricity  is  always  found  in  the  air, 
mof  here1? at"    an(^  aPPears  *°  increase  in  strength  and  quan- 
tity with  the  altitude. 

what  kind  of  It  is  sometimes  different  in  the  lower  re- 
diffuMd*7  gions  from  what  it  is  in  the  upper,  being  posi- 
mospi?eVeVat~  tive  in  one  and  negative  in  the  other  ;  but  in 
the  ordinary  state  of  the  atmosphere,  its  elec- 
tricity is  invariably  positive. 

When  the  sky  is  overcast,  and  the  clouds  are  moving 
in  different  directions,  the  atmosphere  is  subject  to  great 
and  sudden  variations,  rapidly  changing  from  positive 


392  WELLS'S    NATURAL   PHILOSOPHY. 

to  negative,  and  back  again  in  the  space  of  a  few  min- 
utes. 

What  ia  (  The  principal  causes  which  are  supposed  to 

posed  to  occa-     produce   electricity   iti   the   atmosphere    are. 

sion  electricity  .  *  r 

in  the  atmos-  evaporation  from  the  earth's  surface,  chemical 
changes  which  take  place  upon  the  earth's 
surface,  and  the  expansion,  condensation,  and  variation  of 
temperature  of  the  atmosphere  and  of  the  moisture  con- 
tained in  it. 

When  a  substance  is  burning,  positive  electricity  escapes  from  it  into  the 
atmosphere,  while  the  substance  itself  becomes  negatively  electrified.  Thus 
the  air  becomes  the  receptacle  of  a  vast  amount  of  positive  electricity  gener- 
ated in  this  manner. 

When  is  the         The  atmosphere  is  most  highly  charged  with 
atmosphere        electricity  when  hot  weather  succeeds  a  series 

most      highly  V 

charged  with     of  wet  days,  or  wet  weather  follows  a  succes- 

electricity?  .  J    ' 

sion  of  dry  days. 

There  is  more  electricity  in  the  atmosphere  during  the 
cold  of  winter  than  in  the  summer  months. 

Lightning  is  accumulated  electricity,  generally  dis- 
charged from  the  clouds  to  the  earth,  hut  sometimes 
from  the  earth  to  the  clouds. 

who  erst  cs-         764.  The  identity  of  lightning  and  electric- 
4  'of    ity  was  first  established  by  Dr.  Franklin,  at 
nd     Philadelphia,  in  1752. 

The  manner  in  which  this  fact  was  demonstrated  was  as  fol- 
Describe  Frank-  lows  :_Having  made  a  kite  of  a  large  silk  handkerchief  stretch- 
ment.  ed  upon  a  frame,  and  placed  upon  it  a  pointed  iron  wire  con- 

nected with  the  string,  he  raised  it  upon  the  approach  of 
a  thunder-storm.  A  key  was  attached  to  the  lower  end  of  the  hempen 
string  holding  the  kite,  and  to  this  one  end  of  a  silk  ribbon  was  tied, 
the  other  end  being  fastened  to  a  post.  The  kite  was  now  insulated, 
and  the  experimenter  for  a  considerable  time  awaited  the  result  with 
great  solicitude.  Finally,  indications  of  electricity  began  to  appear  on  the 
string;  and  on  Franklin  presenting  his  knuckles  to  the  key,  he  received 
an  electric  spark.  The  rain  beginning  to  descend,  wet  the  string,  increased 
its  conducting  power,  and  vivid  sparks  in  great  abundance  flashed  from 
the  key.  Franklin  afterward  charged  Leyden  jars  with  lightning,  and 
made  other  experiments,  similar  to  those  usually  performed  with  electrical 
machines. 


ATMOSPHERIC    ELECTRICITY.  393 

this         ^he  exPeriment>  as  thus  performed,  was  one  of  great  risk 
experiment  one     and  danger,  since  the  whole  amount  of  electricity  contained  in 


er  tlie  tnuader-cloud  was  Hable  to  pass  from  it,  by  means  of 

the  string,  to  the  earth,  notwithstanding  the  use  of  the  silk 
insulator.* 

From  whatever  cause  electricity  is  present  in  the  air,  the 
cause  of  light-  clouds  appear  to  collect  and  retain  it  ;  and  when  a  cloud  over- 
nine  ?  charged  with  electric  fluid  approaches  another  which  is  under- 

charged, the  fluid  rushes  from  the  former  into  the  latter.  In  a  like  manner, 
the  fluid  may  pass  from  the  cloud  to  the  earth,  and  in  such  cases  elevated 
objects  upon  the  earth's  surface,  as  trees,  steeples,  etc  ,  appear  to  govern  its 
direction. 

"When  a  cloud  highly  charged  with  electricity  is  near  to  the 
circumstances  earth,  the  surface  of  the  earth,  for  a  great  extent,  may  also 

does  lightning     become  highly  charged  by  induction  ;  and  when  the  tension 

pass  from  the  3 

earth    to  the      of  the  electricity  becomes  sufficiently  great,  or  the  two  elec- 


ciouds?  ^rjc  gujfacgg  come  sufficiently  near,  a  flash  of  lightning  not 

unfrequently  passes  from  the  earth  to  the  clouds.     In  this  way  an  equilibrium 
of  the  two  elements  is  restored. 

Lightning  clouds  are  sometimes  greatly  elevated  above  the  surface  of  the 
earth,  and  sometimes  actually  touch  the  earth  with  one  of  their  edges  ;  they 
are,  however,  rarely  discharged  in  a  thunder-storm  when  they  are  more  than 
700  yards  above  the  surface  of  the  earth. 

765.  Lightning  has  "been  divided  into  three 

How        many  .  ,...,. 

kinds  of  light-    kmds,  viz.,  zigzag,  or  chain-lightning,  sheet- 

ning  arc  there? 

lightning,  and  ball-lightning. 

Explain  the  The  zigzag,  or  forked  appearance  of  lightning,  is  believed  to 
diverse  appear^  ^e  occasioned  by  the  resistance  of  the  air,  which  diverts  the 
ance  of  light-  electric  current  from  a  direct  course.  The  globular  form  of 
lightning  sometimes  observed,  is  not  satisfactorily  accounted 
for.  What  is  called  "sheet,"  or-  "heat"  lightning,  is  sometimes  the  reflection 
in  the  atmosphere  of  lightning  very  remote,  or  not  distinctly  visible  ;  but  gen- 
erally this  phenomenon  is  occasioned  by  the  play  of  silent  flashes  of  electricity 
between  the  clouds,  the  amount  of  electricity  developed  not  being  sufficient  to 
produce  any  other  effects  than  the  mere  flash  of  light. 

766.  The  usual  explanation  of  thunder  is, 
cause  of  than?     that  it  is  due  to  a  sudden  displacement  of  the 

particles  of  air  by  the  electrical  current.  Others 
have  supposed  that  the  passage  of  the  electricity  creates 

*  Whon  the  experiment  was  subsequantly  repeated  in  France,  streams  of  electric  fire, 
nine  and  ten  feet  in  length,  and  an  inch  ia  thickness,  darted  spontaneously  with  loud  re- 
ports from  the  end  of  the  string  confining  the  kite.  During  the  succeeding  year,  Prof. 
Eichman  of  St.  Petersburg,  in  making  experiments  somewhat  similar,  and  having  hi» 
apparatus  entirely  insulated,  was  immediately  killed, 

17* 


394  WELLS'S   NATURAL   PHILOSOPHY. 

a  vacuum,  and  that  the  air  rushing  in  to  fill  it  produces 
the  sound.  Every  explanation  that  has  yet  been  offered 
is  somewhat  unsatisfactory. 

The  rolling  of  the  thunder  has  been  ascribed  to  the  effect  of  echo,  but  this 
undoubtedly  is  not  the  only  cause.  The  rolling  of  thunder  is  heard  as  per- 
fectly at  sea  as  upon  land,  but  there  none  of  the  causes  which  are  generally 
supposed  to  produce  echo,  as  mountains,  hills,  buildings,  etc.,  etc.,  are  present. 
Another,  and  perhaps  the  true  reason  is,  that  the  sound  is  developed  by  the 
lightning  in  passing  through  the  air,  and  consequently  separate  sounds  are 
produced  at  every  point  through  which  the  lightning  -passes. 
Where  do  thun  Thunder-storms  prevail  most  in  the  torrid  zone,  and  decrease 
der  storms  in  frequency  toward  either  pole.  In  the  arctic  regions  thunder- 
most  prevail  ?  storms  seldom  or  never  occur.  As  respects  time,  they  are 
most  frequent  in  the  summer  months. 

What  is  called  a  thunder-storm  may  be  considered  to 
be  merely  an  effort  of  nature  to  effect  an  equilibrium  of 
forces  which  have  become  disturbed. 

767.  A  knowledge  of  the  laws  of  electricity  has  enabled 
lightning  con-  man  to  protect  himself  from  its  destructive  influences.  Light- 
ductors  first  niog-rods,  or  conductors,  were  first  introduced  by  Dr.  Frank- 
lin. He  was  induced  to  recommend  their  adoption  as  a  means 
of  protection  to  buildings,  etc.,  from  observing  that  electricity  could  be  quietly 
and  gradually  withdrawn  from  an  excited  surface  by  means  of  a  good  con- 
ductor, which  was  pointed  at  its  extremity. 

As  ordinarily  constructed,  a  lightning-con- 
<juctor  consists  of  a  metal  rod  fixed  in  the 
earth,  running  up  the  whole  height  of  a  building  and  ris- 
ing to  a  point  above  it. 

The  best  metal  that  can  be  used  for  a  light- 
How  should  a  . .,    .  1,1  i 

lightning-rod      ning-rod  is  copper  :    if  iron  is  usexl,  the  rod 

Deconstructed?       ,  .      ,  ,  ,     i      V          ,1  ^i  /         '  /. 

should  not  be  less  than  three  quarters  of  an 
inch  in  diameter.  When  only  one  rod  is  used,  it  should 
be  continuous  from  the  top  to  the  bottom,  and  an  entire 
metallic  communication  should  exist  throughout  its  whole 
length.  This  law  is  violated  when  the  joints  of  the  several 
parts  that  form  the  conductor  are  imperfect,  and  when  the 
whole  is  loosely  put  together. 

The  rod  should  also  be  of  the  same  dimensions  through- 
out. The  rod  is  best  fastened  to  the  building  by  wooden 
supports.  If  there  are  masses  of  metal  about  the  build- 


ATMOSPHERIC    ELECTRICITY.  395 

ing,  as  gutters,  pipes,  etc.,  they  should  be  connected  with 
the  rod  by  strips  of  metal,  and  directly,  if  possible,  with 
the  ground.  The  lower  end  of  the  rod,  where  it  enters 
the  ground,  should  be  divided  into  two  or  three  branches, 
and  turned  from  the  building. 

It  ought  also  to  extend  so  far  below  the  surface  of  the  ground  as  to  reach 
water,  or  earth  that  is  permanently  damp.  It  is,  moreover,  a  good  plan  to 
bury  the  end  of  the  lightning-rod  in  powdered  charcoal,  since  this  pre- 
serves in  a  measure  the  iron  from  rust,  and  facilitates  the  passage  of  the 
electricity. 

A  building  will  be  most  perfectly  protected  when  the  lightning-conductor 
has  several  branches,  with  pointed  rods  projecting  freely  in  the  air  from  dis- 
tant summits  of  the  building,  and  connected  with  the  main  rod. 

Professor  Faraday  advises  that  lightning-conductors  should  be  arranged 
upon  the  inside  of  buildings  rather  than  upon  the  outside. 

what  space  -^-  lightning-conductor  of  sufficient  size  is 
Sdpro?ect?ng"  Believed  to  protect  a  circle  the  diameter  of 
which  is  four  times  the  length  of  that  part  of 
the  rod  which  rises  above  the  building.  Thus,  if  the  rod 
rises  two  feet  above  the  house,  it  will  protect  the  building 
for  (at  least)  eight  feet  all  round. 

A  lightning-conductor  may  be  productive  of  harm  in  two 

iightnin™rod  *     ways ;  if  the  rod  be  broken  or  disconnected,  the  electric  fluid, 

be   productive     bemg  obstructed  in  its  passage,  may  enter  the  building;  and 

if  the  rod  be  not  large  enough  to  conduct  the  whole  current 

to  the  earth,  the  lightning  will  fuse  the  metal  and  enter  the  building. 

A  lightning-conductor  protects  a  building  even  when  no  visible  discharge 
takes  place,  by  attracting  the  electricity  of  an  approaching  cloud,  and  caus- 
ing it  to  pass  off  silently  and  quietly  into  the  earth.  This  process  commences 
as  soon  as  the  cloud  has  approached  a  position  vertically  over  the  rod. 

768.  As  regards  safety  in  a  thunder-storm,  it  is  prudent,  if 

are    safe   and     out  of  doors,  to  avoid  trees  and  elevated  objects  of  every 

misfin  aTlfun"     kind'  wbich  tho  tigering  would  be  likely  to  strike  in  its  pas- 

der-storm  ?  sago  to  the  earth.     A  stream  of  water,  being  a  good  conduc- 

ductor,  should  be  avoided. 

If  within  doors,  the  middle  of  a  carpeted  room  is  tolerably  safe,  provided 
there  is  no  lamp  hanging  from  tho  ceiling.  It  is  prudent  to  avoid  the  neigh- 
borhood of  chimneys,  because  lightning  may  enter  the  room  by  them,  soot 
being  a  good  conductor.  Tor  the  same  reason,  a  person  should  remove  as 
far  as  possible  from  metals,  mirrors,  and  gilt  articles.  Tho  safest  position  that 
can  be  occupied  is  to  lie  upon  a  bed  in  the  middle  of  a  room — feathers  and 
hair  being  excellent  non-conductors.  In  all  cases,  the  position  of  safety  is 
that  in  which  the  body  caa  not  assist  as  a  conductor  to  the  lightning.  Tho 


396  WELLS'S  NATURAL  PHILOSOPHY. 

position  of  surrounding  bodies  must  therefore  be  attended  to,  whether  a  per- 
son be  insulated  or  not. 

The  apprehension  and  solicitude  respecting  lightning  are  proportionate  to 
the  magnitude  of  the  evils  it  produces,  rather  than  the  frequency  of  its  occur- 
rence. The  chances  of  an  individual  being  killed  by  lightning  are  infinitely 
less  than  those  which  he  encounters  in  his  daily  walks,  in  his  occupation,  or 
even  during  his  sleep  from  the  destruction  of  the  house  in  which  he  lodges  by 
fire. 

HOW  are  the  ^69.  The  mechanical  power  exerted  by  light- 
fcSteTficugbfr  nm»  is  enormous  and  difficult  to  account  for. 
mug  accounted  Arago  supposed  that  the  heat  of  the  light- 
ning in  passing  through  any  substance,  in- 
stantly converted  all  the  moisture  contained  in  it  into 
steam  of  a  highly  explosive  character,  and  that  the  great 
mechanical  effects  observed  are  due  to  this  agent  rather 
than  to  the  direct  effect  of  the  electric  current.  A  tem- 
perature that  can  instantly  render  iron  red  hot,  is  known 
to  be  sufficient  to  generate  steam  of  such  an  elastic  force 
that  it  would  overcome  all  obstacles,  and  if  the  water  con- 
tained in  the  pores  of  bodies  is  at  once  converted  into 
steam  of  this  character,  its  force  would  be  capable  of  pro- 
ducing any  of  the  mechanical  effects  witnessed  in  lightning 
discharges. 

Another  theory  supposes  that  the  natural  electricities 
of  non-conducting  bodies  are  forcibly  decomposed  by  the 
presence  of  the  electric  fluid  which  forms  the  lightning, 
and  that  their  violent  separation  forces  every  thing  asun- 
der which  tends  to  confine  them. 

what  is  the  770.  The  phenomenon  of  the  aurora  borealis 
So?abofrelii2?  ™  supposed  to  be  due  to  the  passage  of  electric 
currents  through  the  higher  regions  of  tho 
atmosphere — the  different  colors  manifested  being  pro- 
duced by  the  passage  of  the  electricity  through  air  of  dif- 
ferent densities, 
where  does  the  ^n  tne  northern  hemisphere  the  aurora  al- 

.uroraappear?      wayg  appears  Jn  the    north,  but    in    tllC    SOlltll- 

ern  hemisphere  it  appears  in  the  south  ;  it  seems  to  origin- 
ate at  or  near  the  poles  of  the  earth,  and  is  consequently 


ATMOSPHERIC   ELECTRICITY.  397 

seen  in  its  greatest  perfection  within  the  arctic  and  an- 
tarctic circles.* 

The  aurora  is  not  a  local  phenomenon,  but  is  seen  simultaneously  at  places 
widely  remote  from  each  other,  as  in  Europe  and  America. 

The  general  height  of  the  aurora  is  supposed  to  be  between  one  and  two 
hundred  miles  above  the  surface  of  the  earth ;  but  it  sometimes  appears 
within  the  region  of  the  cloui.is. 

Auroras  occur  more  frequently  in  the  winter  than  in  the  summer,  and  are 
only  seen  at  night.  They  affect  in  a  peculiar  manner  the  magnetic  needle 
and  the  electric  telegraph,  and  as  th«  disturbances  occasioned  in  these  in- 
struments are  noticed  by  day  as  well  as  by  night,  there  can  be  no  doubt  of  the 
occurrence  of  the  aurora  at  all  hours.  The  intense  light  of  the  sun,  however, 
renders  the  auroral  light  invisible  'during  the  day. 

FiG.  331. 


The  accompanying  figure  represents  one  of  the  most  beautiful  of  the  au- 
roral phenomena. 

'  It  has  often  been  asserted,  and  on  good  authority,  that  sounds  have  been 
heard  attending  the  phenomena  of  the  aurora,  like  the  rustling  of  silk,  or  the 
sound  and  crackling  of  a  fire.  On  this  point,  however,  there  is  great  difter- 
ence  of  opinion. 

Auroras  appear  to  be  subject  to  some  variation  in  their  appearance,  extend- 
ing through  a  circle  of  years.  Thus,  from  1705  to  1752,  the  northern  lights 
became  moro  and  more  frequent,  but  after  that  for  a  period  they  were  seen  but 
rarely.  Since  1820  they  have  been  quite  frequent  and  brilliant. 

•  In  the  arctic  and  antarctic  circles,  when  the  sun  is  absent,  the  aurora  appears  with  * 
magnificence  unknown  in  other  regions,  and  affords  light  sufficient  for  many  of  the  ordi- 
nary out-door  employments. 


CHAPTER   XVI. 

GALVANISM. 

what  in  Gai-  771.  ELECTRICITY  excited  or  produced  by 
Sit7?Ele°~  the  chemical  action  of  two  or  more  dissimilar 

substances  upon  each  other   is  termed  Gal- 
vanic, or   Voltaic   Electricity,  and   the   department   of 
physical  science  which  treats  of  this  form  of  electrical 
disturbance  is  called  Galvanism. 
what    simple        The  most  simple  method  of  illustrating  the 

production  of  galvanic  electricity  is  by  placing 

-      a  P^606  °^  S^ver  (as  a  C0m)  On  tne  tOngUC,  and 

a  piece  of  zjnc  underneath.  So  long  as  the 
two  metals  are  kept  asunder  no  effect  will  be  noticed,  but 
when  their  ends  are  brought  together,  a  distinct  thrill  will 
pass  through  the  tongue,  a  metallic  taste  will  diffuse  itself, 
and,  if  the  eyes  are  closed,  a  sensation  of  light  will  be  evi- 
dent at  the  same  moment. 

This  result  is  owing  to  a  chemical  action  which  is  developed  the  moment 
fie  two  metals  touch  each  other.  The  saliva  of  the  tongue  acts  chemically 
upon,  or  oxydizes  a  portion  of  the  zinc,  which  excites  electricity,  for  no  chem- 
ical action  ever  takes  place  without  producing  electricity.  Upon  bringing 
the  ends  of  the  two  metals  together,  a  slight  current  passes  from  one  to  the 
other. 

If  a  living  fish,  or  a  frog,  having  a  small  piece  of  tin- foil  on  its  back,  be 
placed  upon  a  piece  of  zinc,  spasms  of  the  muscles  will  be  excited  whenever 
a  metallic  communication  is  made  between  the  zinc  and  the  tin-foil. 

when  and  how  The  production  of  electricity  by  the  chemi- 
SectriStyVd!t  ca^  action  of  two  metals  when  brought  in  con- 
covered?  tactj  wag  grgt  noticed  by  Galvani,  professor  of 
anatomy  at  Bologna,  Italy,  in  1790. 

His  attention  was  directed  to  the  subject'  in  the  following  manner: — Hav- 
ing occasion  to  dissect  several  frogs,  he  hung  up  their  hind  legs  on  some  cop- 
per hooks,  until  he  might  find  it  necessary  to  use  them  for  illustration,  In. 
this  manner  ho  happened  to  suspend  a  number  of  the  copper  hooks  on  an 


GALVANISM. 


399 


iron  balcony,  when,  to  his  great  astonishment,  the  limbs  were  thrown  into 
violent  convulsions.  On  investigating  the  phenomenon,  he  found  that  the 
mere  contact  of  dissimilar  metals  with  the  moist  surfaces  of  the  muscles  and 
nerves,  was  all  that  was  necessary  to  produce  the  convulsions, 

FiG.  332. 


This  singular  action  of  electricity,  first  noticed  by  Galvani,  may  be  experi- 
mentally exhibited  without  difficulty.  Fig.  332  represents  the  extremities 
of  a  frog,  with  the  upper  part  dissected  in  such  a  way  as  to  exhibit  the  nerves 
of  the  legs,  and  a  portion  of  the  spinal  marrow.  If  we  now  take  two  thin 
pieces  of  copper  and  zinc,  C  Z,  and  place  one  under  the  nerves,  and  the  other 
in  contact  with  the  muscles  of  the  leg,  we  shall  find  that  so  long  as  the  two 
pieces  of  metal  are  separated,  so  long  will  the  limbs  remain  motionless ;  but 
by  making  a  connection,  instantly  the  whole  lower  extremities  will  be  thrown 
into  violent  convulsions,  quivering  and  stretching  themselves  in  a  manner  too 
singular  to  describe.  If  the  wire  is  kept  closely  in  contact,  these  phenomena 
are  of  momentary  duration,  but  are  renewed  every  time  the  contact  is  made 
and  broken. 

Galvani  attributed  these  movements  of  the  muscles  to  a 
kind  of  nervous  fluid  pervading  the  animal  system,  similar  to 
the  electric  fluid,  which  passed  from  the  nerves  to  the  mus- 
cles, as  soon  as  the  two  were  brought  in  communication  with 
each  other,  by  means  of  the  metallic  connection,  in  the  same  way  as  a  dis- 
charge takes  place  between  the  external  and  internal  coatings  of  a  Leyden 
jar.  He  therefore  called  the  supposed  fluid  animal  electricity. 


To  vhat  did 
Gnlvani  attri- 
bute these  phe- 
nomena ? 


400  WELLS'S   NATURAL   PHILOSOPHY. 

What  was  de-  The  exPeriments  of  Galvani  were  repeated  by  Yolta,  en 
termii  ed  by  eminent  Italian  philosopher,  who  found  that  no  electrical  or 
nervous  excitement  took  place  unless  a  communication  be- 
tween the  muscles  and  the  nerves  was  made  by  two  different  metals,  as  cop- 
per and  iron,  or  copper  and  zinc.  He  considered  that  electricity  was  produced 
by  simple  contact  of  the  dissimilar  metals,  positive  electricity  being  evolved  from 
the  one  and  negative  electricity  from  the  other. 

what  is  the  The  true  cause  of  electrical  excitement  occa- 
de^  sioncd  by  the  contact  of  dissimilar  metals  is 
"-  now  folly  ascertained  to  be  chemical  ac- 
ent  metais?  tjon  .  an(j  recent  researches  have  also  proved 
that  no  chemical  action  ever  takes  place  without  the  de- 
velopment of  free  electricity. 

The  electricity  produced  by  chemical  action  has  been 
termed  Galvanic,  or  Voltaic  Electricity,  in  honor  of  Gal- 
vani and  Voita,  who  first  developed  its  phenomena. 
HOW  docs  Rai-  772.  Galvanic  electricity,  or  the  electricity 
Sim0  ordfnSy  developed  by  chemical  action,  differs  from  fric- 
eiectricity?  tional,  or  ordinary  electricity,  chiefly  in  its 
continuance  of  action.  The  electricity  developed  by  fric- 
tion from  a  glass  plate,  or  the  cylinder  of  an  electrical 
machine,  exhibits  itself  in  sudden  and  intermittent  shocks, 
accompanied  with  a  sort  of  explosion  ;  whereas  that  which 
is  generated  by  chemical  action  is  a  steady,  flowing  current. 

The  fundamental  principle  which  forms  the  basis  of  the  science  of  galvanic 
electricity  is  as  follows : 

Any  two  metals,  or  more  generally,  any  two 

Whatprinciple        ,.™      J    ,     ,       ,.  '      ,  '  /.      i 

forms  thr.  basis    different  bodies  which  are  conductors  of  elec- 

of  the   science       .    .    ..  .  ,  ,     .  -.          ,  , 

of      galvanic    tricity.  when  placed  in  contact,  develop  elec- 

electricity?  .    .    .,      ,         i          •      i         .•  •/•  i     \   •    •  ^ 

tricity  by  chemical  action — positive  electricity 
flowing  from  the  metal  which  is  acted  upon  most  power- 
fully, and  negative  electricity  from  the  other. 

In  general,  that  metal  which  is  acted  upon 

What  are  elec-  &     ..      .  ,  ,  .    .  l    , 

tro-poBitivc        most  easily  is  termed  the  electro-positive  metal, 
negative    cie-    or  element ;   and  the  other  the  electro-nega- 
tive metal,  or  element. 

The  electrical  force  or  power  generated  in  this  way  is 
called  the  electro-motive  force. 


GALVANISM. 


401 


How  may  bod- 
ies capable  of 
exciting  elec- 
tro-motive 
forces  be  classi- 
fied? 


773.  Different  bodies  placed  in  contact  manifest  differ- 
ent electro-motive  forces,  or  develop  different  quantities 
of  electricity. 

Bodies  capable  of  developing  electricity  by  contact  may  be 
arranged  in  a  series  in  such  a  manner  that  any  one  placed  in 
contact  with  another  holding  a  lower  place  in  the  series,  will 
receive  the  positive  fluid,  and  the  lower  one  the  negative  fluid  ; 
and  the  more  remote  they  stand  from  each  other  in  the  order 
of  the  series,  the  more  decidedly  will  the  electricity  bo  developed  by  their 
contact. 

The  most  common  substances  used  for  exciting  galvanic  electricity  may  bo 
arranged  in  such  a  series  as  follows: — zinc,  lead,  tin,  antimony,  iron,  brass, 
copper,  silver,  gold,  platinum,  black  lead  or  graphite,  and  charcoal. 

Thus,  zinc  and  lead,  when  brought  in  contact,  will  produce  electricity,  but 
it  will  be  much  less  active  than  that  produced  by  the  union  of  zinc  and  iron, 
or  the  same  metal  and  copper,  and  the  last  less  active  than  zinc  and  platinum 
or  zinc  and  charcoal. 

774.  In  the  production  of  galvanic  electricity  for  practical 
purposes,  it  is  necessary  to  have  a  combination  of  tiiree  dif- 
ferent conductors,  or  elements,  one  of  which  must  be  solid 
and  one  fluid,  while  the  third  may  be  efcher  solid  or  fluid. 

The  process  usually  adopted  is  to  place  between  two  plates 
of  different  kinds  of  metal  a  liquid  capable  of  exciting  some  chemical  action 
on  one  of  the  plates,  while  it  has  no  action,  or  a  different  action  upon  the 
other.  A  communication  is  then  formed  between  the  two  plates. 

When  two  metals  capable  of  exciting  elec- 
tricity are  so  arranged  and  connected  that  the 
positive  and  negative  electricities  can  meet  and  flow  in 
opposite  directions,  they  are  said  to  form  a  galvanic  cir- 
cuit, or  circle. 

A  very  simple,  and 
Describe  a  sim-  J 

pie  Galvanic  at  the  same  time  an  ac- 
Battery.  ^ ye  gaivanic  circuit  may 

be  formed  by  an  arrangement  as  repre- 
sented in  Fig.  333.  C  and  Z  are  thin 
plates  of  copper  and  zinc  immersed  in  a 
glass  vessel  containing  a  very  weak  so- 
lution of  sulphuric  acid  and  water. 
Metallic  contact  can  be  made  between 
the  plates  by  wires,  X  and  W,  which 
are  soldered  to  them.  If  now  the  wires 
are  connected,  as  at  Y,  a  galvanic  cir- 
cuit will  be  formed ;  positive  electricity 
passing  from  the  zinc  through  the  liq- 


What  is  the 
practical  meth- 
od of  exciting 
galvanic  elec- 
tricity '( 


What  is  a.  Gal- 
vanic Circuit  ? 


FIG.  333. 


402  WELLS'S   NATURAL   PHILOSOPHY. 

uid,  to  the  copper,  and  from  the  copper  along  the  conducting-wires  to  the 
zinc,  as  indicated  by  the  arrows  in  the  figure.  A  current  of  negative  elec- 
tricity at  the  same  time  traverses  the  circuit  also,  from  tho  copper  to  the 
zinc,  in  a  direction  precisely  reversed. 

Such  an  arrangement  is  called  a  simple  galvanic  battery. 

what  are  the  The  two  metals  forming  the  elements  of  the 
?an!c  bitter^ ?"  battery  are  generally  connected  by  copper 
wires  ;  the  ends  of  these  wires,  or  the  terminal 
points  of  any  other  connecting  medium  used,  are  called  the 
poles  of  the  battery. 

Thus,  when  zinc  and  copper  plates  are  used,  tho  end  of  the  wire  conveying 
positive  electricity  from  the  copper  would  be  the  positive  pole,  and  the  end  of 
the  wire  conveying  negative  electricity  from  the  zinc  plate  would  be  the 
negative  pole.  Faraday  describes  the  poles  of  the  battery  as  the  doors  by 
which  electricity  enters  into  or  passes  out  of  the  substance  suffering  decom- 
position, and  in  accordance  with  this  view  he  has  given  to  the  positive  pole 
tho  name  of  anode,  or  ascending  way,  and  to  the  negative  pole  the  name  of 
cathode,  or  descending  way. 

At  what  point        The  manifestations  of  electricity  will  be  most 
isf  theiecctricity    apparent  at  that  point  of  the  circuit  where  the 
manifested?       two  currents  of  positive  and  negative  electricity 
meet. 

When  is  a  ir  When  the  two  wires  connecting  the  metal  plates  of  a  bat- 
cuit  said  to  be  tery  are  brought  in  contact,  the  galvanic  circuit  is  said  to  be 

closed.  No  sign  of  electrical  excitement  is  then  visible ;  the 
action,  nevertheless,  continues.  The  opposite  electricities  collected  at  the 
poles,  in  particular,  neutralize  each  other  perfectly  on  meeting ;  every  trace 
of  electricity  must  therefore  vanish,  as  when  a  Leydcn  jar  is  discharged,  if  a 
fresh  quantity  were  not  continually  produced  by  the  pairs  of  plates.  If  tho 
wires  which  conduct  the  two  electricities  be  slightly  disconnected,  a  spark 
will  be  observed  at  the  point  of  interruption. 

.  In  the  formation  of  a  galvanic  circuit,  by  the  employment 

theory  of  the  of  two  metals  and  a  liquid,  the  chemical  action  which  gives 
^?vaUnici0neiecf  rise  to  the  el.ectricit7  takes  Place  through  a  decomposition  of 
tricTty.  the  liquid.  It  is,  therefore,  essential  to  the  formation  of  an 

active  galvanic  circuit,  that  the  liquid  employed  should  be  ca- 
pable of  being  decomposed..  .-Water  is  most  conveniently  applicable  for  this 
purpose.  When  a  plate  of  zinc  and  copper  are  immersed  in  water,  the  ele- 
ments of  the  water,  oxygen  and  hydrogen,  are  separated  from  each  other,  in 
consequence  of  the  greater  attraction  which  the  oxygon  has  for  the  zinc.  The 
oxygen,  therefore,  unites  with  the  zinc,  and  by  so  doing  produces  an  altera- 
tion in  the  electrical  condition  of  the  metal.  The  zinc  communicating  its  nat- 
ural share  of  electricity  to  the  liquid,  becomes  negatively  electrified.  Tho 


GALVANISM.  403 

copper  attracting  the  same  electricity  from  the  Jiquid,  becomes  positively 
electrified,  ;  at  the  same  time  the  hydrogen,  which  is  the  other  element  of 
the  water,  is  also  attracted  to  the  copper,  and  appears  in  minute  bubbles  upon 
its  surface.  If  the  two  metal  plates  be  now  connected  with  metallic  wires, 
positive  electricity  will  flow  from  the  copper  and  negative  electricity  from  the 
zinc,  and  by  the  union  of  these  two  an  electric  current  will  be  formed.* 

With  water  alone  and  two  metals,  the  quantity  of  electricity  excited  is  very 
small,  but  by  the  addition  of  a  small  quantity  of  some  acid,  the  excitement  is 
greatly  increased. 

.  Although  two  metal  plates  are  employed  in  the  arrangement 

necessity  of  two  described,  only  one  of  them  is  active  in  the  excitement  of  elec- 
vTnTc  circuit?1"  tricitv'  the  other  Plate  serving  merely  as  a  conductor  to  collect 
the  force  generated.  A  metal  plate  is  generally  used  for  this 
purpose,  because  metals  conduct  electricity  much  better  than  other  substances 
exposing  an  equal  surface  to  the  fluids  in  which  they  are  immersed ;  but  other 
conductors  may  be  used,  and  when  a  proportionately  larger  surface  is  ex- 
posed to  compensate  for  inferior  conducting  power,  they  answer  as  well,  and 
in  some  instances  better,  than  metal  plates.  Thus  charcoal  is  very  often  em- 
ployed in  the  place  of  copper,  and  a  very  hard  material  obtained  from  the  in- 
terior of  gas  retorts,  called  graphite,  is  considered  one  of  the  best  conductors. 

Two  metals  are  not  absolutely  essential  to  the  formation  of  a  simple  gal- 
vanic circuit.  A  current  may  be  obtained  from  one  metal  and  two  liquids, 
provided  the  liquids  are  such  that  a  stronger  chemical  action  takes  place  on 
one  side  of  the  metal  plate  than  on  the  other. 

In  some  electric  batteries  also,  two  metals  and  two  dissimilar  liquids  are 
employed. 

775.  The  electricity  developed  by  a  simple 

How  may  gal-  *_  r  * 

vanic  action  be    galvanic   circuit,  whether  it  be  composed  of 

increased?  -i        T      '  •  -i  -, 

two  metals  and  a  liquid,  or  any  other  combin- 
ation, is  exceedingly  feeble.  Its  power  can,  however,  be 
increased  to  any  extent  by  a  repetition  of  the  simple  com- 
binations. 

•  The  terms  "  electric  fluid"  and  "  electric  current,"  which  are  frequently  employed  iu 
describing  electrical  phenomena,  are  calculated  to  mislead  the  student  into  the  supposi- 
tion that  electricity  is  known  to  be  a  fluid,  and  that  it  flows  in  a  rapid  stream  along  the 
wires.  Such  terms,  it  should  be  understood,  are  founded  merely  on  an  assumed  analogy 
of  the  electric  force  to  fluid  bodies.  The  nature  of  that  force  is  unknown,  and  whether  its 
transmission  be  in  the  form  of  a  current,  or  by  vibrations,  or  by  any  other  means,  is  un- 
determined. 

In  a  discussion  which  took  place  some  years  since  at  a  meeting  of  the  British  Associa- 
tion for  the  Advancement  of  Science,  respecting  the  nature  of  electricity,  Professor  Fara- 
day expressed  his  opinion  as  follows  : — "  There  was  a  time  when  I  thought  I  knew  some- 
thing about  the  matter  ;  but  the  longer  I  live,  and  the  more  carefully  I  study  the  subject, 
the  more  convinced  I  am  of  my  total  ignorance  of  the  nature  of  electricity." 

"  After  such  an  avowal  as  this,"  says  Mr.  Bakewell,  "  from  the  most  eminent  electrician 
of  the  age,  it  is  almost  useless  to  say  that  any  terms  which  seem  to  designate  the  form  of 
electricity  are  merely  to  be  considered  as  convenient  conventional  expressions." 


404 


WELLS'S   NATURAL   PHILOSOPHY. 


The  first  attempt  to  increase  FIG.  334. 

pile  of  Volta.0  *ne  P°wer  of  a  galvanic  circuit 
by  increasing  the  number  of 
the  combinations,  was  made  by  Volta.  He 
constructed  a  pile  of  zinc  and  copper  plates 
with  a  moistened  cloth  interposed  between 
each.  He  commenced  with  a  zinc  plate,  upon 
which  he  placed  a  copper  plate  of  the  same 
size,  and  on  that  a  circular  piece  of  cloth  pre- 
viously soaked  in  water  slightly  acidulated. 
On  the  cloth  was  laid  another  plate  of  zinc, 
then  copper,  and  again  cloth,  and  so  on  in  suc- 
cession, until  a  pile  of  fifty  series  of  alternate 
metal  plates  and  moistened  cloths  was  formed, 
the  terminal  plate  of  the  series  at  one  end  being 
copper  and  at  the  other  end  zinc.  A  metallic 
wire  attached  to  the  highest  copper  plate  will 
constitute  the  positive  polo,  and  another  to  the  lowest  zinc  plate  the  negative 
pole  of  such  a  series. 

Fig.  334  represents  Volta's  arrangement  of  metal  plates  and  wet  cloths, 
with  the  metallic  wires,  which  constitute  the  poles. 

Such  combinations  are  denominated  Voltaic   Piles,  or 
Voltaic  Batteries,  and  very  often  Galvanic  Batteries. 

As  two  different  metals  and  an  interposing  liquid  are  generally  employed 
for  this  purpose,  it  has  been  usual  to  call  these  combinations  pairs  or  elements ; 
so  that  the  battery  is1  said  to  consist  of  so  many  pairs  or  elements,  each  pair 
or  element  consisting  of  two  metals  and  a  liquid. 

776.  Voltaic  piles  or  batteries  have  FIG.  335. 

stances14  have  been  composed  and  constructed  in 
voltaic  piles  a  great  variety  of  forms,  by  combin- 
ed W  ing  together  in  a  series  various  sub- 
stances which  excite  electricity  when 
acted  upon  chemically. 

Thus,  they  have  been  constructed  entirely  of  veg- 
etable substances,  without  resorting  to  the  use  of 
any  metal,  by  placing  discs  of  beet-root  and  walnut- 
wood  in  contact.  "With  such  a  pile,  and  a  leaf  of 
grass  aa  a  conductor,  convulsions  in  the  muscles  of  a 
dead  frog  are  said  to  have  been  produced.  Other 
experimentalists  have  formed  voltaic  piles  wholly 
of  animal  substances. 

A  perfectly  dry  voltaic  pile,  known 
from  its  inventor  as  Zamboni's  Pile, 
may  be  formed  of  sheets  of  gilded 
paper  and  sheet  zinc.     If  several  thousands  of  these 


GALVANISM. 


405 


be  packed  together  in  a  glass  tube,  so  that  their  similar  metallic  faces  shall 
all  look  the  same  way,  and  be  pressed  tightly  together  at  each  end  by  metallic 
plates,  it  will  be  found  that  one  extremity  of  the  pile  is  positive  arid  the 
other  negative.  Such  a  series  will  last  more  than  twenty  years,  but  it  re- 
quires as  many  as  10,000  pairs  to  afford  sparks  visible  in  daylight,  and  to 
charge  the  Leydeu  jar. 

Fig.  335  represents  a  pair  of  theso  piles,  so  arranged  as  to  produce  what 
has  been  called  a  perpetual  motion.  .  Two  piles,  P  N,  are  placed  in  such  a 
position  that  their  poles  are  reversed,  and  between  them  a  light  pendulum, 
vibrating  on  an  axis  and  insulated  on  a  glass  pillar.  This  pendulum  is  alter- 
nately attracted  to  one  and  then  to  the  other,  and  thus  rings  two  little  bella 
connected  with  the  positive  and  negative  poles. 

The  galvanic  batteries  in  practical  use  at  the  present  time  differ  consider- 
ably in  form  and  efficiency,  but  the  principle  of  construction  in  all  is  the  same 
as  that  of  the  original  voltaic  pile. 

A   very   effective 
Describe      the 
trough  battery,     arrangement  known 

as  the  trough  bat- 
tery, is  represented  in  Fig.  336. 
This  consists  of  a  trough  of  wood 
divided  into  water-tight  cells,  or 
partitions,  each  cell  being  arranged 
to  receive  a  pair  of  zinc  and  copper 
plates.  The  plates  are  attached  to 
a  bar  of  wood,  and  connected  wi!h 
one  another  by  metallic  wires,  in 
such  a  way  that  every  copper  plato 
is  connected  with  the  zinc  plate  of 
the  next  cell.  The  battery  is  excited  by  means  of  dilute  sulphuric  acid  poured 
into  the  cells,  and  the  current  of  electricity  is  directed  by  wires  soldered  to  the 
extreme  plates.  When  the  battery  is  not  in  use  the  plates  may  be  raised  from 
the  trough  by  means  of  the  wooden  bar. 

The  battery  by  which  Sir  Humphrey  Davy  effected  his  splendid  chemical 
discoveries  was  of  this  form,  and  consisted  of  two  thousand  double  plates  of 
copper  and  zinc,  each  plate  having  a  surface  of  thirty-two  square  inchoa 
Now,  however,  by  improved  arrangements,  wo  can  produce  with  tea  or 
twenty  pairs  of  plates,  effects  every  way  superior. 

Fio.  337. 


FIG.  336. 


fSfr'.frg 


406 


WELLS  S    NATURAL   PHILOSOPHY. 


Describe  Smee's 
battery. 


In  other  and  more  efficient  compound  galvanic  circuits,  the  exciting  liquid 
is  placed  in  a  series  of  separate  cups,  or  glasses,  arranged  in  a  circle,  or  in 
parallel  lines.  Each  cup  contains  one  zinc  and  one  copper  plate,  not  imme- 
diately in  connection  with  each  other,  but  every  zinc  plate  of  one  cup  is  con- 
nected with  the  copper  plate  of  the  preceding,  by  a  copper  baud,  or  wire. 
This  arrangement  is  represented  in  Fig.  337,  the  copper  plate,  and  the  direc- 
tion of  the  positive  current  being  indicated  by  the  sign  +,  and  the  zinc  plate 
and  the  direction  of  the  negative  current  by  the  sign  — . 

The  simplest  form  of  galvanic  battery  at  present  used  ia 
that  invented  by  Mr.  Smee,  and  known  as  Smee's  battery 
(See  Fig.  338.)  It  consists  of  a  plate  of  silver  coated  with 
platinum,  suspended  between  two  plates  of  zinc,  z  z,  the  sur- 
faces of  which  last  have  been  coated  with  mercury,  or  amal- 
gamated, as  it  is  called.*  The  three  are  attached  to  a  wooden 
bar,  which  serves  to  support  the  whole  in  a  tumbler,  Gr,  par- 
tially filled  with  a  weak  solution  of  sulphuric  acid  and  water. 
The  wires,  or  poles  for  directing  the  current  of  electricity  are 
connected  with  the  zinc  and  platinum  plates  by  small  screw- 
cups,  S  and  A. 

What    is    the         Another  form  of  battery,  called  the  sulphate 
sulphate  of  cop- ,   of  copper  battery,  from  the  fact  that  a  solution 
per  battery?        of  suiphate  Of  COpper  (blue  vitriol)  is  used  as 
the  exciting  liquid,  is  represented  by  Fig.  339.     It  consists  of  two  concentric 
cylinders  of  copper  tightly  soldered  to  a  copper  bottom, 
and  a  zinc  cylinder,  Z,  fitting  in  between  them.     The 
zinc  cylinder,  when  let  down  into  the  solution,  is  pre- 
vented from  touching  the  copper  by  means  of  three 
pieces  of  wood  .or  ivory,  shown  in  the  figure.     Two 
screw-cups  for  holding  the  connecting  wires   are   at- 
tached, one  to  the  outer  copper  cylinder,  and  the  other 
to  the  zinc. 

The  principal  imperfection  of  the  gal- 
vanic battery  is  the  want  of  uniformity 
in  its  action.  In  all  the  various  forms 
the  strength  of  the  electric  current  ex- 
cited continually  diminishes  from  the  moment  the  battery 
action  commences.  In  the  sulphate  of  copper  battery,  especially,  the  power 
is  reduced  to  almost  nothing  in  a  comparatively  brief  space  of  time.  -  This  i* 
is  chiefly  owing  to  the  circumstance  that  the  metallic  plates  soon  become 
coated  with  the  products  of  the  chemical  decomposition,  the  result  .tf  the 
chemical  action,  whereby  the  electricity  is  developed. 

This  difficulty  is  obviated,  in  .a  great  degree,  by  the  use  of  a  diaphragm,  or 
porous  partition,  between  the  two  metallic  plates,  which  allows  a  free  contact 

*  It  is  found  that  by  coating  the  zinc  with  mercury,- the  waste  of  the  zinc  is  greatly 
diminished.  It  is  not  well  understood  in  what  way  the  mercury  contributes  to  this  effect. 
We  have  a  parallel  to  it  in  the  rubber  of  the  electrical  machine,  >hich,  when  coated  with 
an  amalgam  of  zinc  and  tin,  aeta  with"  greater  efficiency  than  under  any  other  circum 


FlG.  339. 


What  is  the 
principal  im- 
perfection of 
the  galvanic 
battery  ? 


GALVANISM. 


40T 


of  the  liquid  on  each  side,  within  its  pores,  but  prevents  the  solid  products 
of  decomposition  from  passing  fr6m  one  plate  to  the  other. 
Descr       Da  Daniel's  constant    battery,   constructed  according   to   this 

iei's     constant    principle,  and  represented  in  Fig.  340,  maintains  an  effective 
battery.  galvanic    action    longer 

than  any  other ;  a  is  a  hollow  cylinder 
of  copper;  z,  a  solid  rod  of  amalgam- 
ated zinc;  and  e,  a  porous  tube  of 
earthenware  separating  the  two. 
Diluted  sulphuric  is  placed  hi  the 
porous  tube,  and  a  saturated  solution 
of  sulphate  of  copper  in  the  copper 
cylinder. 

What  is  the  On6  °f  the  m°St  6ffi- 
construction  of  cient  batteries  is  that 
Grove's  bat-  known  as  Grove's  bat- 
tery, from  its  inventor,  and  is  the  form  generally  used  for 
telegraphing  and  for  other  purposes  in  which  powerful  galvanic  action  is  re- 
quired. It  consists  of  a  plain  glass  tumbler,  in  which  is  placed  a  cylinder  of 
amalgamated  zinc,  with  an  opening  on  one  side  to  allow  a  free  circulation  of 
the  liquid  Within  this  cylinder  is  placed  a  porous  cup,  or  cell,  of  earthen- 
ware, in  which  is  suspended  a  strip  of  platinum  fastened  to  the  end  of  a  zinc 
arm  projecting  from  the  adjoining  zinc  cylinder.  The  porous  cup  containing 
the  platinum  is  filled  with  strong  nitric  acid,  and  the  outer  vessel  containing 

the  zinc  with  weak  sulphuric 

FiG.  341.  acid.     ^g.  341  represents  a 

series  of  these  cups,  arranged 
to  form  a  compound  circuit, 
with  their  terminal  poles,  P 
and  Z.  This  form  of  battery 
is  objectionable  on  account 
of  the  corrosive  character  of 
the  acids  employed,  and 
the  deleterious  vapors  that 
arise  from  it  when  in  ac- 
tion. 

777.  The  electricity  evolved  by  a  single  gal- 
circle  is  great  in  quantity,  but  weak  in 
electricity?        intensity. 

These  two  qualities  may  be  compared  to  heat  of  different  temperatures.  A 
gallon  of  water  at  a  temperature.of  100°  has  a  greater  quantity  of  heat  than  a 
pint  at  200° ;  but  the  heat  of  the  latter  is  more  intense  than  that  of  the  former. 

The  electricity,  on  the  contrary,  produced 
r  friction,  or  that  of  the  electrical  machine, 

electricity?        js  smau  {Q  quantity,  but  of  high  tension,  or 

intensity. 


What  is  the  dis- 


wh«t4a  the  dis- 


408  WELLS'S    NATURAL   PHILOSOPHY. 

Illustrate  the  Frictional  electricity  is  capable  of  passing  for  a  considerable 
differences  be-  distance  through  or  over  a  non-conducting  or  insulating  sub- 
electricities^0  stance?  which  galvanic  electricity  can  not  do.  Thus,  the  spark 
from  a  prime  conductor  will  leap  toward  a  conducting  sub- 
stance for  some  distance  through  the  air,  which  is  a  non-conductor;  but  if  a 
current  of  galvanic  electricity  is  resisted  by  the  slightest  insulation,  or  the  in- 
terposition of  some  non-conducting  substance,  the  action  at  once  stops.  Gal- 
vanic electricity  will  traverse  a  circuit  of  2,000  miles  of  wire,  rather  than  make 
a  short  circuit  by  overleaping  a  space  of  resisting  air  not  exceeding  one  hun- 
dredth part  of  an  inch.  Frictional  electricity,  on  the  other  hand,  will  force  a 
passage  across  a  considerable  interval,  in  preference  to  taking  a  long  circuit 
through  a  conducting  wire,  or  at  least  the  greater  portion  of  it  will  pass 
through  the  air,  though  some  part  of  the  charge  will  always  traverse  the  wire. 

Frictional  electricity  produces  very  slight  chemical  or  heating  effects ;  gal- 
vanic electricity  produces  very  powerful  effects. 

A  proper  and  simple  arrangement  of  a  zinc  plate  and  a  little  acidulated 
water,  will  produce  as  much  electricity  in  three  seconds  of  time  as  a  Leyden 
jar  battery  charged  with  thirty  turns  of  a  large  and  powerful  plate  electrical 
machine  in  perfect  action.  The  shock  received  by  transmitting  this  quantity 
of  galvanic  electricity  through  the  animal  system  would  be  hardly  perceptible, 
but  received  from  a  Leyden  jar,  would  be  highly  dangerous,  and  perhaps 
fatal.  A  grain  of  water  may  be  decomposed  and  separated  into  its  two  ele- 
ments, oxygen  and  hydrogen,  by  a  very  simple  galvanic  battery,  in  a  very 
short  time;  but  800,000  such  charges  of  a  Leyden  jar  battery,  as  above  re- 
ferred to,  would  be  required  to  supply  electricity  sufficient  to  accomplish  the 
same  result.  Such  a  quantity  of  electricity  sent  forth  from  a  Leyden  jar 
would  be  equal  to  a  very  powerful  flash  of  lightning. 

upon  what  does  The  quantity  of  electricity  excited  iu  a  gal- 
JaJic^dectSd-  vamc  circuit  is  directly  proportional  to  the 
ty  depend?  amount  of  chemical  action  that  takes  place — 
as  between  the  zinc  and  the  acid.  By  increasing  the 
amount  of  surface  exposed  to  chemical  action,  we  there- 
fore increase  the  quantity  of  electricity  evolved. 

Hence,  gigantic  plates  have  been  constructed  for  the  purpose  of  obtaining 
an  immense  quantity. 

The  intensity  of  the  electricity  evolved  de- 

Upon  what  does  ,  ,  ,  ,,      ,  ,. 

intensity    de-    pends  upon  the  number  of  plates,  and  is  great- 
est when  the  voltaic  pile  is  made  up  of  a  great 
number  of  small  plates. 

Supposing  an  equal  amount  of  surface  of  copper  and  zinc  employed,  the 
shock,  and  other  indications  of  a  strong  charge,  would  be  greater  if  it  were 
cut  up  into  many  small  circles,  than  if  it  formed  a  few  large  ones.  But  the 
actual  quantity  of  excitement  would  be  greatest  with  the  large  plates. 


GALVANISM.  409 

may  roi-  ?78.  When  the  wire  from  one  end  of  a  vol- 
*a^c  battery  is  connected  with  the  wire  from 
renewed?  ^.]ie  0ppOSite  end,  voltaic  action  instantly  com- 
mences ;  and  it  as  instantaneously  ceases  when  the  con- 
nection is  interrupted.  The  rapidity  with  which  the  elec- 
tric circuit  may  be  completed  and  broken  has  no  ascertained 
limit ;  nor  does  it  appear  to  be  controlled  by  resistance 
caused  by  traversing  miles  of  wire. 

779.  The  most  ordinary  effects  produced  by 

What  are  the  .    •    • .  »          t  i  • 

most  ordinary    the  developed  electricity  of  a  large  galvanic 

effects  of   gal-  •/  f       °    , 

vanic  eiectrici.  battery,  are  the  production  of  sparks  and  bril- 
liant flashes  of  light,  the  heating  and  fusing 
of  metals,  the  ignition  of  gunpowder  and  other  inflam- 
mable substances,  and  the  decomposition  of  water,  saline 
compounds,  and  metallic  oxyds. 

Heat  is  evolved  whenever  a  galvanic  cur- 

When  does  gal-  .  ° 

vanic  eiectrici-    rent  passes  over  a  conducting  body,  the  amount 

ty evolve  heat?  • f.  J '    .  ,    . 

of  which  will  depend  on  the  quantity  and  in- 
tensity of  the  electricity  transmitted,  and  upon  the  re- 
sistance which  the  body  offers  to  the  passage  of  the  cur- 
rent. 

The  metals  differ  greatly  in  their  conducting  power.  Thus,  if  we  link 
together  pieces  of  copper,  iron,  silver,  and  platinum  wire,  and  pass  a  galvanic 
current  along  them,  they  will  be  found  to  be  unequally  heated,  the  platinum 
being  the  most,  and  the  copper  the  least. 

The  easiest  method  of  showing  by  experiment  the  heating 
helung^eff^  power  of  the  galvanic  current  is  to  connnect  the  poles  of  a 
of  galvanic  battery  by  means  of  a  fine  platinum  wire.  If  the  wire  is  very 
illustrated?  *  long  &  ma7  become  hot:  shorten  it  to  a  certain  extent,  and 

it  will  become  red-hot ;  shorten  it  still  more,  and  it  will  be- 
come white-hot,  and  finally  melt.  If  such  a  wire  is  carried  through  a  small 
quantity  of  salt  water  on  a  watch-glass,  the  liquid  will  boil ;  if  through  alco- 
hol, ether,  or  phosphorus,  they  will  be  inflamed ;  if  through  gunpowder,  it  will 
be  exploded. 

What  ractical  Tllis  Power  lias  been  applied  to  the  purpose  of  firing  blasts, 
application  has  or  mines  of  gunpowder,  an  operation  which  may  be  effected 
°f  wit^  e(*ual  faciutJ  under  water.  The  process  is  as  follows : — 

The  wires  from  a  sufficiently  powerful  battery  are  connected 
by  a  piece  of  fine  platinum  wire,  which  is  placed  in  a  mass  of  gunpowder  con-> 
tained  in  a  cavity  of  a  rock,  or  inclosed  in  a  vessel  beneath  the  surface  of 
water.  The  wire  may  be  of  any  length,  but  the  moment  connection  is  made 

18 


410  WELLS'S   NATURAL   PHILOSOPHY. 

with  the  battery  the  current  passes,  renders  the  platinum  red-hot,  and  ex- 
plodes the  the  powder.* 

How  ma  the  "^  ^e  S1"63*08'  artificial  heat  man  has  yet  succeeded  in  pro- 
greatest  artifi-  ducing  has  been  through  the  agency  of  the  galvanic  battery. 
"reduced*?  be  "^  ^ie  metaH  including  platinum,  which  cau  not  be  fused 
by  any  furnace  heat,  are  readily  melted.  Gold  burns  with  a 
blueish  light,  silver  with  a  bright  green  flame,  and  the  combustion  of  tho 
other  metals  is  always  accompanied  with  brilliant  results.  Ail  the  earthy 
minerals  may  be  liquefied  by  being  placed  between  the  poles  of  a  sufficiently 
large  battery.  Sapphire,  quartz,  slate,  and  lime,  are  readily  melted ;  and 
-the  diamond  itself  fuses,  boils,  and  becomes  converted  into  coal. 

HOW  are  the  780.  The  luminous  effects  of  the  galvanic 
fcSJD°of  the  battery  are  no  less  remarkable  than  its  heating 
teiy'manifest-  effects.  A  very  small  voltaic  arrangement  is 
ed?  sufficient  to  produce  a  spark  of  light  every 

time  the  circuit  is  closed  or  opened.  If  the  two  ends  of 
wires  proceeding  from  the  opposite  poles  of  a  battery  are 
brought  nearly  together,  a  bright  spark  will  pass  from  one 
to  the  other,  and  this  takes  place  even  under  water,  or  in 
a  vacuum. 

now  may  the  The  most  splendid  artificial  light  known  is 
Mt*ficiainii!jfbt  produced  by-fixing  pieces  of  pointed  charcoal 
be  produced?  ^o  fae  wires  connected  with  opposite  poles  of  a 
powerful  galvanic  battery,  and  bringing  them  within  a  short 
distance  of  each  other.  The  space  between  the  points  is 
occupied  by  an  arch  of  flame  that  nearly  equals  in  dazzling 
brightness  the  rays  of  the  sun. 

This  light,  which  is  termed  tho  electric  light,  differs  from 

decTric°e8light     a11  °}her  forms  of  artificial  light,  inasmuch  as  it  is  independent 

differ  from  all     of  ordinary  combustion.     The  charcoal  points  appear  to  suffer 

light*  ?artificU1     no  change,  and  the  light  is  equally  strong  and  brilliant  in  a 

vacuum,  and  in  such  gases  as  do  not  contain  oxygen,  where 

•  In  the  course  of  the  construction  of  a  railway  recently  in  England,  it  became  neces- 
gary  to  detach  a  large  mass  of  rock  from  a  cliff  on  the  sea-coast  in  order  to  avoid  the  ex- 
pense of  a  long  tunnel.  To  have  done  this  by  the  direct  application  of  human  labor  and 
the  ordinary  operations  of  blasting,  would  have  been  attended  with  an  immense  expendi- 
ture of  time  and  money.  It  was  accordingly  resolved  to  blow  it  up  with  gunpowder, 
ignited  by  the  galvanic  battery.  Nine  tons  of  po-vrder  were  accordingly  deposited  in  cham- 
bers at  from  50  to  70  feet  from  the  face  of  the  cliff,  and  fired  by  a  conducting  wire  connected 
with  a  powerful  battery,  placed  at  1 ,000  feet  from  the  mine.  The  explosion  detached 
600,000  tons'  weight  of  chalk  from  the  cliff.  It  was  proved  that  this  might  have  been 
equally  effected  at  the  distance  of  3,000  feet.  This  bold  experiment  savod  eight  months' 
labor  and  $50,009  expense. 


GALVANISM.  411 

all  other  artificial  lights  would  be  extinguished.  It  may  even  be  produced 
under  water.  To  excite  the  electricity,  however,  which  occasions  this  light, 
zinc  or  some  other  m*  tal  must  be  oxydized,  or  what  is  the  same  thing  burnt, 
the  same  as  oil  in  our  lamps,  or  coal  in  the  gas  retorts  for  the  production  of 
other  species  of  artificial  light. 

The  effects  of  the  galvanic  battery  upon  the 

"What  are   the  ••  i  *»   j  i  i 

physiological  nerves  and  muscles  ol  the  animal  system  are 
vSi?  electric-  of  the  same  character  as  those  produced  by 
|ity?  ordinary  electricity. 

On  grasping  the  two  ends  of  the  connecting  wires  of  a  battery  of  some 
force  with  wet  hands,  a  peculiar  tremor  will  be  felt  in  the  joints  of  the  arm 
and  hand,  accompanied  by  a  slight  contortion  of  the  muscles,  and  increasing 
to  a  violent  shock.  This  shock  is  repeated  every  time  a  contact  between  the 
hand  and  the  wire  is  broken  and  renewed.  The  concussion  of  the  nerves  of 
the  body  is,  therefore,  produced  by  the  entrance  and  exit  of  the  currents  of 
electricity ;  for  they  evidently  must  pass  through  the  body  the  moment  it  forma 
the  connecting  link  between  the  two  poles. 

By  a  particular  arrangement,  the  circuit  may  be  closed  or  interrupted  at 
pleasure,  and  in  such  a  manner  that  the  current  may  be  made  to  pass  alter- 
nately through  the  wires  and  the  body ;  the  latter  being  thus  exposed  to  a 
series  of  shocks  which  are  considered  particularly  adapted  for  the  cure  of 
diseases  arising  from  the  injury  or  derangement  of  the  nervous  system.  It  is, 
moreover,  a  highly  valuable  remedy  in  cases  of  suffocation,  drowning,  paraly- 
sis, etc. ,  and  numerous  arrangements  have  been  at  various  times  proposed 
for  the  construction  of  medico-galvanic  machines. 

The  effects  of  galvanic  electricity  on  bodies  recently  deprived  of  life  is  very 
remarkable,  and  it  was  through  an  accidental  observance  of  its  action  upon  a 
dead  fro.?  that  galvanism  was  discovered.  By  connecting  the  muscles  and 
nerves  of  recently-killed  animals  with  the  poles  of  a  battery,  many  of  the 
movements  of  life  may  be  produced.  Some  remarkable  experiments  of  this 
character  were  made  some  years  since  upon  the  body  of  a  man  recently 
executed  for  murder  at  Glasgow,  in  Scotland.  The  voltaic  battery  em- 
ployed consisted  of  270  pairs  of  plates,  four  inches  square.  On  applying 
one  pole  of  the  battery  to  the  forehead  and  the  other  to  the  heel,  the 
muscles  are  described  to  have  moved  with  fearful  activity,  so  that  rage, 
anguish,  and  despair,  with  horrid  smiles,  wero  exhibited  upon  the  counten- 
ance. 

781.  Galvanic  electricity  is  a  powerful  agent  in  effecting  chemical  decom- 
positions, and  in  its  application  to  such  purposes,  it  is  most  practically  useful. 

can  galvanic  When  a  current  of  galvanic  electricity  is 
fecfrichLicIi  made  to  pass  through  a  compound  conducting 
decomposition?  substance,  its  tendency  is  to  decompose  and 
separate  it  into  its  constituent  parts. 


412  WELLS'S   NATURAL  PHILOSOPHY. 

How  may  wa-         Thus,  water  is  composed  of  two  gases,  oxygen  and  hydro- 
ter  be  decom-     gen  united  together.     When  the  wires  connecting  the  poles 
of  a  galvanic  battery  are  placed  in  water,  and  a  sufficiently 
strong  current  made  to  pass  through  them,  the  water  is  decomposed,  the 
FIG  342  hydrogen  being  given  out  at  the  negative  pole  of  the 

battery,  and  the  oxygen  at  the  positive  pole.  Pig. 
342  represents  a  form  of  apparatus  by  which  this 
experiment  can  be  performed  in  a  very  satisfactory 
manner.  It  consists  of  two  tubes,  0  and  H,  sup- 
ported vertically  in  a  small  reservoir  of  water, 
and  two  slips  of  platinum,  p  p,  which  can  be  con- 
nected with  the  poles  of  a  voltaic  battery,  passing 
in  at  the  open  end  of  the  tubes.  When  communi- 


cation is  effected  between  the  platinum  slips  and  a 
\     battery  in  action,  gas  rapidly  rises  in  each  tube  and 

collects  in  the  upper  part.  In  that  tube  which  is  in 
connection  with  the  positive  pole  of  the  battery  oxygen  accumulates,  and  in 
the  other  hydrogen.  And  it  will  be  noticed  that  the  quantity  of  the  latter  is 
equal  to  twice  the  quantity  of  the  former  gas,  since  water  contains  by  volumo 
twice  as  much  hydrogen  as  it  does  oxygen. 

What  is  the  Tlie  exPlanation  of  tnis  phenomenon  may  be  briefly  given 
theory  of  the  as  follows : — All  atoms  of  matter  are  regarded  as  originally 
aX™PofinJil-  charSed  with  either  positive  or  negative  electricity.  In  tho 
vanic  elec-  case  of  water,  hydrogen  is  the  electro-positive  element  and 
oxygen  the  electro-negative  element.  It  has  been  already 
shown  that  bodies  in  opposite  electrical  states  are  attracted  by  each  other. 
Hence,  when  the  poles  of  a  galvanic  battery  are  immersed  in  water,  the  nega- 
tive pole  will  attract  the  positive  hydrogen,  and  the  positive  pole  the  negative 
oxygen.  If  the  attractive  force  of  the  two  electricities  generated  by  the  bat- 
tery is  greater  than  the  attractive  force  which  unites  the  two  elements,  oxygen 
and  hydrogen,  together  in  the  water,  the  compound  will  be  decomposed.  Upon 
the  same  principle  other  compound  substances  may  be  decomposed,  by  em- 
ploying a  greater  or  less  amount  of  electricity.  In  this  way  Sir  Humphrey 
Davy  made  the  discovery  that  potash,  soda,  lime,  and  other  bodies,  were  not 
simple  in  their  nature,  as  had  previously  been  supposed,  but  compounds  of  a 
metal  with  oxygen.  - | 

What       ntit  **82'  Recent  experiments  have  shown  that  the  electricity 

of  electricity  is  which  decomposes,  and  that  which  is  evolved  by  the  decom- 
decompose  *»  Position  of  a  certain  quantity  of  matter,  are  alike.  Thus,  water 
substance  ?  is  composed  of  oxygen  and  hydrogen  ;  now,  if  the  electrical 

power  which  holds  a  grain  of  water  in  combination,  or  which 
causes  a  grain  of  oxygen  and  hydrogen  to  unite  in  the  right  proportions 
to  form  water,  could  be  collected  and  thrown  into  a  voltaic  current,  it 
would  be  exactly  the  quantity  required  to  produce  the  decomposition 
of  a  grain  of  water  or  the  liberation  of  its  elements,  oxygen  and  hy- 
drogen. 


GALVANISM.  413 

what  i«  an  ?83.  For  convenience  in  certain  experi- 
Eiectrode?  ments,  the  ends  of  the  copper  wires  connect- 
ing the  poles  of  the  galvanic  battery  are  frequently 
terminated  with  thin  strips  of  platinum,  which  are  called 
Electrodes.  The  platinum  slip  connected  with  the  posi- 
tive pole  forms  the  positive  electrode,  and  that  with  the 
negative  pole,  the  negative  electrode. 

Platinum  is  used  for  the  reason,  that  in  employing  the  battery  for  effecting 
decompositions,  it  is  frequently  necessary  to  immerse  the  ends  of  the  con- 
ducting wires  in  corrosive  liquids,  and  this  metal  generally  is  not  affected  by 
them. 

what  is  Eiec-  784.  Electro-metallurgy,  or  electro-typing,  is 
tro-metaiiurgyf  t^e  art  or  process  of  depositing,  from  a  metal- 
lic solution,  through  the  agency  of  galvanic  electricity,  a 
coating  or  film  of  metal  upon  some  other  substance.* 
upon  what  is  The  process  is  based  on  the  fact,  that  when 
based ?process  a  galvanic  current  is  passed  through  a  solu- 
tion of  some  metal,  as  of  sulphate  of  copper 
(sulphuric  acid  and  oxyd  of  copper),  decomposition  takes 
place  ;  the  metal  is  separated  in  a  metallic  state,  and 
attaches  itself  to  the  negative  pole,  or  to  any  substance 
that  maybe  attached  to  the  negative  pole  ;  while  the 
oxygen  or  other  substance  before  in  combination  with  the 
metal,  goes  to,  and  is  deposited  on  the  positive  pole. 

In  this  way  a  medal,  a  wood-engraving,  or  a  plaster  cast,  if  attached  to  the 
negative  pole  of  a  battery,  and  placed  in  a  solution  of  copper  opposite  to  the 
positive  pole,  will  be  covered  with  a  coating  of  copper ;  if  the  solution  con- 
tains gold  or  silver  instead  of  copper,  the  substance  will  be  covered  with  a 
coating  of  gold  or  silver  in  the  place  of  copper. 

The  thickness  of  the  deposit,  providing  the  supply  of 
the  metallic  solution  be  kept  constant,  will  depend  on  the 
length  of  time  the  object  is  exposed  to  the  influence  of  the 
battery. 

In  this  way,  a  coating  of  gold  thinner  than  the  thinnest  gold-leaf  can  be 
laid  on,  or  it  may  be  made  several  inches  or  feet  in  thickness,  if  desired. 
The  usual  arrangement  for  conducting  the  electrotype  process  is  represented 

•  The  general  name  of  electro-metallurgy  includes  all  the  rarious  processes  and  result* 
which  different  inventors  and  manufacturers  have  designated  as  galvano-plastic,  electro- 
plastic,  galrano-type,  electro-typing,  and  electro-plating  and  gilding. 


414 


WELLS'S  NATURAL  PHILOSOPHY. 


by  Fig.  343.  It  consists  of  a  trough  of  wood,  or  an  earthen  vessel,  containing 
the  solution,  the  decomposition  of  which  is  desired— for  example,  sulphate  of 
copper.  Two  wires,  one  connected  with  the  positive,  and  the  other  with  the 
negative  pole  of  a  battery,  Q.  are  extended  along  the  top  of  the  trough,  and 
supported  on  rods  of  dry  wood,  B  and  D.  The  medal,  or  other  article  to  be 
coated,  is  attached  to  the  negative  wire,  and  a  plate  of  metallic  copper  to  the 
positive  wire.  When  both  of  these  are  immersed  in  the  liquid,  the  action 
commences — the  sulphate  of  copper  is  decomposed — the  copper  being  de- 
posited on  the  medal,  and  the  liberated  oxygen  on  the  copper  plate.  As  the 
withdrawal  of  the  metal  from  the  solution  goes  on,  the  copper  plate  attached 
to  the  positive  pole  undergoes  corrosion  by  the  sulphuric  acid  which  is  liber- 
ated and  attracted  to  it,  and  sulphate  of  copper  is  formed.  This,  dissolving  in 
the  liquid,  maintains  it  at  a  constant  strength.  When  the  operator  judges 
that  the  deposit  on  the  medal  is  sufficiently  thick,  he  removes  it  from  the 
trough,  and  detaches  the  coating.  The  deposit  is  prevented  from  adhering  to 
the  medal  by  rubbing  its  surface  in  the  first  instance  with  oil,  or  black-lead, 
and  if  it  is  desired  that  any  part  of  the  surface  should  be  left  uncoated,  that 
portion  is  covered  with  wax,  or  some  other  non-conductor. 

PlQ.  343. 


In  this  way  a  most  perfect  reversed  copy  of  the  medal  is  obtained, — that  is, 
the  elevations  and  depressions  of  the  original  are  reversed  in  the  copy.  To 
obtain  a  fac-simile  of  the  original,  the  electrotype  cast  is  subjected  to  a  repe- 
tition of  the  process. 

In  general,  it  is  found  more  convenient  to  mold  the  object  to  bo  repro- 
duced in  wax,  or  Plaster  of  Paris.  The  surface  of  this  cast  is  then  brushed 
over  with  black-lead  to  render  it  a  conductor,  and  the  metal  deposited  directly 
upon  it.  The  deposit  obtained  will  then  exactly  resemble  the  original  ob- 
ject. 

The  pages  and  engravings  in  the  book  before  the  reader  are  illustrations  of 
the  perfection  and  practical  application  of  the  electrotype  process.  The  en- 
gravings were  first  cut  upon  wood-blocks,  and  then,  with  the  ordinary  type, 
formed  into  pages.  Casts  of  the  whole  in  wax  were  next  made,  and  an  elec- 


GALVANISM.  415 

trotype  coating  of  copper  deposited  upon  them,  and  from  the  copper  plates 
so  formed  the  book  was  printed.  The  great  advantage  of  this  is,  that  the 
copper  being  harder  than  the  ordinary  type  metal,  is  more  durable,  and  re- 
sists the  wear  of  printing  from  its  surface  for  a  longer  period. 

The  improvement  effected  by  electro-metallurgy  in  engrav- 
electrotype  ing  is  very  great.  When  a  copper  plate  is  engraved,  and  im- 
pressioris  printed  off  from  it,  only  the  first  few,  called  " proof 
impressions,  "possess  the  fineness  of  the  engraver's  delineation. 
The  plate  rapidly  wears  and  becomes  deteriorated.  But  by  the  electrotype 
process,  the  original  plate  can  at  once  be  multiplied  into  a  great  many  plates 
as  good  as  itself,  and  an  unlimited  number  of  the  finest  impressions  pro- 
cured. 

In  this  way  the  map  plates  of  the  Coast  Survey  of  the  United  States,  some 
of  which  require  the  labor  of  the  engraver  for  years,  and  cost  thousands  of 
dollars,  are  reproduced — the  original  plate  being  never  printed  from. 

One  of  the  simplest  illustrations  of  metallic  deposit  by  electro-chemical  ac- 
tion is  afforded  by  the  following  experiment : — Put  a  piece  of  silver  in  a  glass 
containing  a  solution  of  sulphate  of  copper,  and  into  the  same  glass  insert  a 
piece  of  zinc.  No  change  will  take  place  in  either  metal  so  long  as  they  are  kept 
apart ;  but  as  soon  as  they  touch,  the  copper  will  be  deposited  upon  the  sil- 
ver, and  if  it  be  allowed  to  remain,  the  part  immersed  will  be  completely 
covered  with  copper,  which  will  adhere  so  firmly  that  mere  rubbing  alone  will 
not  remove  it 

HOW  does  the  785.  When  two  metals  which  are  positive 
™£!8of.ffJ£  and  negative  in  their  electrical  relations  to 
their  durability?  Qa(fa  other,  are  brought  in  contact,  a  galvanic 
action  takes  place  which  promotes  chemical  change  in  the 
positive  metal,  but  opposes  it  in  the  negative  metal. 

Thus,  when  sheets  of  zinc  and  copper  immersed  in  dilute 
What  are  illns-  rt 

trations  of  this     acid  touch  each  other,  the  zinc  oxydizes  or  rusts  more,  and  the 

principle  ?  copper  less  rapidly,  than  without  contact.     Iron  nails,  if  used 

in  fastening  copper  sheathing  to  vessels,  rust  much  quicker  than  when  in  other 
situations,  not  in  contact  with  the  copper.  The  reason  is,  that  the  contact  of 
the  two  metals  excites  galvanic  action,  which  causes  the  iron  to  rust  speedily, 
but  protects  the  copper. 

what  is  gai-  What  is  called  galvanized  iron,  is  iron  cov- 
vanizediron?  erec|  entirely,  or  in  part,  with  a  coating  of 
zinc.  The  galvanic  action  between  the  two  oxydizes  the 
zinc,  but  protects  the  iron  from  rust. 

Copper,  when  immersed  in  sea- water,  rapidly  wastes  by  the 
How  did  Davy  .     ,        .          _  xl  ,     ,    ... 

attempt  to  pro-     chemical  action  of  the  oxygen  dissolved  m  sea-water ,  but  if 

tect  the  sheath-  ft  ^e  brought  in  contact  with  zinc,  or  some  metal  that  is  more 
*V*a  corrosion  ?  electro-positive  than  itself,  the  zinc  will  undergo  a  rapid 


416  WELLS'S   NATURAL   PHILOSOPHY. 

change,  and  the  copper  will  be  preserved.  Sir  Humphrey  Davy  attempted  to 
apply  this  principle  to  the  protection  of  the  copper  sheathing  of  ships,  by 
placing  at  intervals  over  the  copper  small  strips  of  zinc.  The  experiment 
was  tried,  and  A  piece  of  zinc  as  large  as  a  pea  was  found  adequate  to  pre- 
serve forty  or  fifty  square  inches  of  copper ;  and  this  wherever  it  was  placed, 
whether  at  the  top,  bottom,  or  middle  of  the  sheet,  or  under  whatever  form 
it  was  used.  The  value  of  the  application  was,  however,  neutralized  by  a 
consequence  which  had  not  been  foreseen.  The  protected  copper  bottom 
rapidly  acquired  a  coating  of  sea-weeds  and  shell-fish,  whose  friction  on  the 
water  became  a  serious  resistance  to  the  motion  of  the  vessel,  and  it  was  dis- 
covered that  the  bitter,  poisonous  taste  of  the  copper  surface,  when  corroded, 
acted  in  preventing  the  adhesion  of  living  objects.  The  principle,  however, 
has  been  applied  wilh  success  to  protect  the  iron  pans  used  in  evaporating 
sea-water. 


CHAPTER    XVII. 

THERMO-ELECTRICITY. 

what  is  Ther-  ^86.  IF  two  dissimilar  metallic  bars  be  sol- 
mo-eiectricityf  fteTe&  together,  and  heated  at  the  point  of 
junction,  an  electric  current  will  circulate  through  them, 
and  may  be  carried  off  by  connection  with  any  good  con- 
ductor. Electricity  thus  generated  or  developed  is  called 
Thermo-electricity. 

Thus,  if  two  bars,  one  of  German  silver  and  the  other  of  brass,  as  repre- 
sented in  Fig.  344  (the  dark  one  being  the  brass),  be  heated  at  their  junction, 
FiG.  344.  an  electric  current  will  flow  in  the  direction  of  tho 

arrows  from  the  German  silver  to  the  brass. 

Different  degrees  of  temperature,  also,  in  the  same 
metal,  will  occasion  an  electric  current  to  flow  from 
the  colder  to  the  warmer  portions. 

The  properties  of  thermo-electricity 
are  the  same  as  those  of  ordinary  electricity. 

The  metals  best  adapted  for  showing  its  effects  are 
German  silver,  bismuth,  brass,  iron,  and  antimony. 

Hoir  ar  ther  Thermo-electric  batteries  of  considerable  power  may  be  con- 
mo-electric  bat-  structed  by  combining  together  alternate  plates  of  German  silver 
•trueted?  C°n"  and  brass»  or  bismuth  and  antimony,  thick  cards  of  pasteboard 
being  so  placed  between  the  plates,  that  a  contact  of  the 
metals  is  prevented,  except  at  the  ends.  Such  a  battery,  represented  by  Fig. 


MAGNETISM. 


417 


345,  may  be  made  to  develop  electricity  by  heating  FIG.  345. 

one  end  of  the  bundle,  or  pile  of  plates. 

By  binding  together  two  bars  of  bismuth  and 
antimony,  an  electric  current  can  be  proved  to  circu- 
late with  the  slightest  variation  of  temperature. 

A  series  of  slender  bars  of  these  two  metals,  ar- 
ranged as  a  thermo-electric  battery,  is  far  more  sen- 
sitive to  heat  than  the  most  delicate  thermometer ; 
so  that  the  heat  radiated  from  the  hand  brought  near 
to  one  end  of  the  battery  is  sufficient  to  excite  an  appreciable  amount  of  elec- 
tricity. 

Pig.  346  represents  the  construction  of  such'a  battery.     It  consists  of  thirty- 
p      34g  six  delicate  bars  of  bismuth  and  antimony, 

alternately  connected  at  their  extremities 
and  packed  in  a  case,  the  ends  of  which 
are  removed  in  the  figure  to  show  the 
bars.     The  area  of  such  a  battery  is  not 
quite  one  half  an  inch.     A  represents  a 
conical  reflector,  used  to  concentrate  rays 
of  heat  in  experimenting. 
It  has  been  also  found  that  when  hot  water  mixes  with  cold  water,  that 
electricity  is  produced ;  the  hot  liquor  being  positive  and  the  cold  negative. 


CHAPTER    XTIII. 


MAGNETISM. 

whatisanat-  787.  A  NATURAL  magnet,  sometimes  called 
urai  magnet?  a  loadstone,  is  an  ore  of  iron,  known  as  the 
protoxyd  of  iron,  or  magnetic  oxyd  of  iron,  which  is  ca- 
pable of  attracting  other  pieces  of  iron  to  itself. 

Natural  magnets  are  by  no  means  rare ;  they 
are  found  in  many  places  in  the  United  States, 
and  in  Arkansas,  especially,  an  ore  of  iron  pos- 
sessing remarkably  strong  attractive  powers  is 
very  abundant. 

The  magnetic  ore  is  usually  of  a  dark  color, 
and  possesses  but  little  metallic  luster.  If  a 
piece  of  this  ore  be  dipped  in  iron  filings,  or 
brought  in  contact  with  a  number  of  small 
18* 


j.      2 ,  i- 


418  WELLS'S  NATURAL  PHILOSOPHY. 

needles,  they  will  adhere  to  the  extremities  of  the  magnet,  as  is  represented 
in  i'ig.  347. 

can  a  magnet  .  When  a  natural  magnet  is  brought  near  to, 
or  in  contact  with  a  piece  of  soft  iron  or  steel, 
it  communicates  its  attractive  properties,  and 
renders  the  iron  a  magnet.  In  doing  so,  it  loses  none  of 
its  original  attractive  influence. 

what  are  arti-  Bars  of  iron  or  steel  which  by  contact  with 
ficiai  magnets?  natllrai  magnets,  or  by  other  methods,  have 
acquired  magnetic  properties,  are  termed  artificial  magnets. 

For  all  practical  purposes,  artificial  magnets  are  used  in  preference  to  nat- 
ural magnets,  and  can  be  made  more  powerful. 

The  attractive  force  of  magnets  has  received 

Define         the 

meaning  of  the    the  name  of  MAGNETIC  FORCE,  and  that  de- 

terms  magnetic  _ 

force  and  mag-    partment  of  science  which  treats  of  magnets 

netism-  1,1-  i  •  •  T  •  i       iT/r 

and    their   properties   is   denominated   MAG- 
KETISM. 

This  designation  must  not  be  confounded  with  Animal  Magnetism,  a  term 
which  has  been  adopted  to  designate  a  certain  influence  which  one  person 
may  exercise  over  another  by  means  of  the  will. 

what  are  the  ^^'  e  ^tTSLctivQ  power  of  the  magnet  is 
poies  of  a  mag-  not  diffused  uniformly  over  every  part  of  its 
surface,  but  resides  principally  at  opposite 
points  or  extremities  of  its  surface.  These  points  are 
termed  poles. 

Between  the  regions  of  greatest  attraction,  a  point  may 
be  found  where  the  attractive  influence  wholly  disappears. 

.  "When  a  bar  magnet  is  rolled  in  iron  filings,  the  filings  attach  themselves 
to  the  magnet  in  the  manner  represented  in  Fig.  348,  and  in  this  way  clearly 
indicate  the  location  of  the  magnetic  force. 

FiG.  348. 


In  a  steel  magnet,  the  actual  poles,  or  points  of  greatest  magnetic  intensity, 
are  not  exactly  at  the  ends,  but  at  a  distance  of  about  one  tenth  of  an  inch 
from  each  extremity.  . 


MAGNETISM.  419 

in  what  posi-        789.  When  a  magnet  is  supported  in  such 

netn7reday^l"      ft    WaV    aS    tO    m°Ve    *'reely>  &  wil1    rest    °nty  m 

pended  rest  i  one  position,  viz.,  with  its  poles,  or  extremi- 
ties  directed  nearly  north  and  south. 

If  drawn  aside  from  this  position,  it  will  continue  to 
vibrate  backward  and  forward,  until  it  again  rests  in  the 
same  position. 

The  pole,  or  extremity  of  the  magnet  that 

What  are   the  i  •  fi  • 

north  and  south  constantly  points  toward  the  north,  is  called 
net?  the  North  Pole,  and  the  one  that  points  to- 

ward the  south,  the  South  Pole  of  the  magnet. 

790.  That  property  of  a  magnet  which  will 
what  i*oi«»s-    cause  it,  when  suspended  freely,  to  constantly 

°owerfrective    *urn   *^e  same   Par*  toward  the  north  pole, 
and  the  opposite  part  toward  the  south  pole 
of  the  earth,  is  termed  magnetic  polarity,  or  directive 
power. 

When  a  magnet,  being  free  to  move,  places 

When  is  a  mag-  °          '  °  '  f 

net  said  to  tra-    itself  after  deflection  in  a  nearly  north  and 

verse?  .      .  * 

south  line,  it  is  said  to  traverse. 

The  attractive  force  of  the  loadstone,  or  natural  magnet,  can  not  be  consid- 
ered as  of  any  great  amount.  Native  magnets,  in  their  rude  state,  will  sel- 
dom lift  their  own  weight,  and,  with  some  rare  exceptions, 'their  power  is 
limited  to  a  few  pounds. 

what  IB  th  *^*  When  two  bodies  possessing  magnetic 
general  law  of  properties  are  brought  near,  or  in  contact  with 
tractions  and  each  other,  the  like  poles  will  repel,  and  the 

repulsions?  ...  *L 

unlike  attract  each  other. 

Thus,  the  north  or  the  south  poles  of  two  magnets  re- 
pel each  other ;  but  the  north  pole  of  the  one  will  attract 
the  south  pole  of  the  other. 

792.  Magnetism  may  be  excited  most  read- 
in   vhat   sub-      ....  J 

etances    may    ily  m  iron  and  steel.     In  steel  the  magnetic 

magnetism   be          J  . 

most  easily  ex-    property,  when  induced,  remains  permanent; 
but  soft  iron  loses  its  power  as  soon  as  it  is  re- 
moved from  the  influence  of  the  exciting  magnet.     Brass, 
nickel,  and  cobalt  may  also  be  rendered  magnetic. 


420 


WELLS'S   NATURAL   PHILOSOPHY. 


In  what  form 
are  artificial 
magnets  con- 
structed ? 

FlG.  349. 


Recent  investigations  have  shown  that  the  influence  of  magnetism,  which 
•was  once  supposed  to  be  wholly  restricted  to  iron  and  its  compounds,  is  al- 
most as  pervading  and  wide-extended  as  that  of  electricity.  The  emerald, 
the  ruby,  and  other  precious  stones,  the  oxygen  of  the  air,  glass,  chalk,  bone, 
wood,  and  many  other  substances,  are  more  or  less  susceptible  to  magnetic 
influence.  This  influence,  however,  is  perceptible  only  by  the  nicest  tests, 
and  under  peculiar  circumstances. 

Artificial  magnets  of  iron  or  steel  may  be  of  any  required 
form,  or  of  almost  any  dimensions.  For  general  purposes, 
they  are  limited  to  straight  bars. 

"When  a  piece  of  iron  not  magnetic  is  brought  in  contact 
with  a  common  magnet,  it  will  be  attracted  by  either 
pole ;  but  the  most  powerful  attraction  takes  place  when 
both  poles  can  be  applied  to  the  surface  of  the  piece  of 
iron  at  once.  The  magnetic  bars  are  for  this  purpose 
bent  somewhat  into  the  shape  of  the  letter  U,  and  aro 
termed  horse-shoe  magnets. 

Several  of  these  are  frequently  joined  together  with 
their  similar  poles  in  contact ;  they  then  constitute  a 
compound  magnet,  and  are  very  powerful,  either  for 
lifting  weights  or  charging  other  magnets. 

For  the  purpose  of  distinguishing  between  the  two 
poles  of  an  artificial  magnet,  the  end  of  the  bar  which  is  designated  as  the 
north  pole  is  generally  marked  with  a  -f-  or  with  the  letter  N. 

if  we  break  an  If  we  break  a  magnet  across  the  middle, 
£e?ficwhatmo?-  eaca  fragment  becomes  converted  into  a  per- 
curs?  fec{.  magnet;  the  part  which  originally  had  a 

north  pole  acquires  a  south  pole  at  the  fractured  end,  and 
the  part  which  originally  had  a  south  pole,  gets  a  north  pole. 


FIG.  350. 


Thus,  if  the  bar  N  S,  Fig.  350,  be 
broken  in  the  center,  each  of  the  fractured 
ends  will  exhibit  a  polar  state,  as  perfect 
as  the  entire  magnet ;  the  fractional  end  s 
becoming  a  south  and  n  a  north  pole,  al- 
though at  this  middle  point,  where  n  and 
a  join,  no  magnetism  could,  before  the  breaking,  have  been  detected. 

If  we  divide  up  a  magnet  to  the  extreme  degree  of  mechanical  fineness 
possible,  each  particle,  however  small,  will  be  a  perfect  magnet. 

The  properties  of  a  magnet  are  not  at  all  affected  by 
the  presence  or  absence  of  air  ;  but  its  influence  is  as 
great  in  a  vacuum  as  in  any  other  situation. 

Heat  weakens  the  power  of  a  magnet,  and  a  white  heat 
destroys  it  entirely. 


MAGNETISM.  421 

where  doesthe  793.  The  magnetic  power  of  an  iron  or  steel 
™a?ebody01r?  magnet  appears  to  reside  wholly  upon  the  sur- 
Bide?  face,  and  to  circulate  about  it. 

be™  Tendered  To  render  a  bar  of  steel  magnetic,  the  north 
magnetic?  p0]e  Of  a  niagnet  is  placed  on  the  center  of  a 
bar  of  steel  and  repeatedly  drawn  over  it  toward  one  ex- 
tremity ;  the  other  half  is  subjected  to  a  similar  treat- 
ment with  the  south  pole  of  the  magnet ;  the  bar  is  thus 
rendered  magnetic,  and  only  loses  this  property  when 
strongly  heated. 

A  bar  of  soft  iron  becomes  magnetic  by  sim- 
iro.T  magnet-    pie  contact  with  a  magnet,  but  the  effect,  as 

before  stated,  is  not  permanent. 
May  iron  be        It  is  not  necessary  that  absolute  contact 

rendered  mag-  ;  .  J  . 

netic  by  indue-    should  take  place  between  a  bar  of  soft  iron 

tion?  r 

and  a  magnet,  in  order  to  render  the  iron 
magnetic  ;  but  whenever  a  magnet  is  brought  near  to  a 
piece  of  iron  in  any  shape,  the  latter  is  rendered  magnetic 
by  the  influence  of  the  former.  To  this  phenomenon  the 
name  of  induction  has  been  given,  and  the  distance  through 
which  this  effect  can  take  place  is  called  the  magnetic  at- 
mosphere. 

-,       »ri  Thus,  let  a  bar  of  soft  iron,  B,  as  in  Fig.  351,  be 

*      .       brought  near  to  a  magnet,  M,  whose  poles,  north  and 

p ~ 1  south,  are  indicated  by  IS"  and  S.     By  induction,  the 

L  *  \  bar  will  be  rendered  magnetic,  the  end  of  the  bar  to- 
ward the  north  pole  of  the  magnet  constituting  its 
south  pole,  and  the  other  end  the  north  pole. 

In  all  cases,  where  either  pole  of  a  magnet  is  brought 
near  to,  or  in  contact  with  bodies  capable  of  acquiring 
magnetism,  the  part  which  is  nearest  to  the  pole  of 
:  the  magnet  acquires  a  polarity  opposite,  while  the  re- 

mote extremity  becomes  a  pole  of  the  same  kind  ;  hence  the  attraction  of  a 
magnet  for  iron,  is  simply  the  attraction  of  one  pole  of  a  magnet  for  the  oppo- 
site pole  of  another. 

How  may  the  Tne  general  effect  of  magnetization  by  induction  may  be 
phenomena  of  clearly  exhibited  by  bringing  a  powerful  magnet  near  to  a 
duc^on^e  ex-  piece  of  soft  iron,  as  a  large  key,  when  it  will  be  found  that 
hibited  ?  the  large  key  will  support  several  smaller  ones ;  but  as  soon 

as  the  body  inducing  the  magnetic  action  is  removed,  they  all  drop  oft 


422  WELLS'S   NATURAL  PHILOSOPHY. 

can  the  earth        Magnetism  may  be  also  induced  in  a  "bar  of 

induce  magnet-      jr0n  jjy  the  action  of  the  Caith. 
ism  7  *^ 

Most  iron  bars  and  rails,  as  the  vertical  bars 
of  windows,  that  have  stood  for  a  considerable  time  in  a 
perpendicular  position,  will  be  found  to  be  magnetic. 

If  we  suspend  a  bar  of  soft  iron  sufficiently  long  in  the  air,  it 
trXom?*  U1Uof  wil1  graduall7  become  magnetic ;  and  although  when  it  is  first 
magnetism  in-  suspended  it  points  indifferently  in  any  direction,  it  will  at 
earth*?  **  ""*  ^st  point  north  and  south. 

If  a  bar  of  iron,  such  as  a  kitchen  poker,  which  has  been 
found  to  be  devoid  of  magnetism,  is  placed  with  one  end  on  the  ground, 
slightly  inclined  toward  the  north,  and  then  struck  one  smart  blow  with  a 
hammer  upon  the  upper  end  it  will  acquire  polarity,  and  exhibit  the  attractive 
and  repellent  properties  of  a  magnet. 

Does  magnetic  Magnetic  attraction  can  be  made  to  exert 
tenT^rough  *ts  influence  through  glass,  paper,  and  solid 
other  bodies?  an^  n^^  substances  generally  which  are  not 
capable  of  acquiring  magnetic  influence  in  the  ordinary 
manner. 

If  a  horse-shoe  magnet  be  placed  under- 

FIG.  352.  neath  a  sheet  of  paper  which  has  iron 

filings  sprinkled  over  its  surface,  the  fil- 
ings, upon  the  approach  of  the  magnet, 
will  arrange  themselves  in  great  regularity 
in  lines  diverging  from  the  poles  of  the 
magnet,  in  curves,  and  extending  from 
the  one  pole  to  the  other,  as  is  repre- 
sented in  Fig.  352.  The  numerous  frag- 
ments of  iron,  being  rendered  magnets  by 
induction,  have  their  unlike  poles  fronting 
each  other,  and  they  therefore  attract  one 

another,  and  adhere  in  the  direction  of  their  polarities,  forming  what  are  termed 

magnetic  curves. 
If  a  plate  of  iron  is  caused  to  intervene  between  the  magnet  and  the  under 

surface  of  the  paper,  the  magnetic  influence  is  almost  entirely  cut  off. 

DO  artificial  794.  Magnets,  if  left  to  themselves,  gradu- 
thef?etprojer!  a^J?  an(^  m  a  8Pace  °f  time  varying  with  the 
ties?  hardness  of  the  metal  composing  them,  lose 

their  magnetic  properties,  from  the  recombination  of  their 
separate  fluids. 

This  is  prevented  by  keeping  their  poles  united  by 


MAGNETISM.  423 

what   is    an    means  of  a,  soft  iron  bar  called  an  Armature, 
Armature?      represented  at  A,  Fig.  349. 
\  This  becoming  magnetic  by  induction,  reacts  upon  the 
magnetism  in  the  poles  of.  the  magnetic  bar,  and  tends  to 
increase  rather  than  diminish  their  intensity. 
what  is  the        ^he  lifting  or  sustaining  power  of  magnets 
varies  very   materially.      The  most  powerful 
that  we  are  acquainted  with  are  capable  of 
sustaining  twenty-six  times  their  own  weight. 
NOW  does  the        The  law  of  magnetic  attraction  and  repul- 
SticattraSS    s^on  *s  *ne  same  as  that  of  gravitation  ;  that 
varV  ?repulsioa    *s?  ^iese  forces  increase  in  the  same  proportion 
as  the  square  of  the  distance  from  the  center 
of  attraction  or  repulsion  diminishes. 

.       d.r       t  795.  The  various  phenomena  of  magnetism  have  been  ac- 

what  theory  are  counted  for  by  supposing  that  all  bodies  susceptible  of  magnet- 
nomena6  Pac-  ism  are  Pervaded  b7  a  subtle  imponderable  fluid,  which  is  corn- 
counted  for?  pound  in  its  nature,  and  consists  of  two  elements,  one  called 
the  austral,  or  southern  magnetism,  and  the  other  the  boreal, 
or  northern  magnetism.  Each  of  these,  like  positive  and  negative  electrici- 
ties, repel  their  own  kind,  and  attract  the  opposite  kind. 

When  a  body  pervaded  by  the  compound  fluid  is  in  its  natural  state  and 
not  magnetic,  the  two  fluids  are  in  combination  and  neutralize  each  other. 
When  a  body  is  magnetic,  tlie  fluid  which  pervades  it  is  decomposed,  the  austral 
fluid  being  directed  to  one  extremity  of  the  body,  and  the  boreal  to  the  other. 

.  Iron  and  steel  are  easily  rendered  magnetic,  because  the  fluids  which  per- 
vade them  can  be  easily  decomposed  by  the  action  of  other  magnets.  In 
iron,  the  separation  of  the  two  kinds  of  magnetism  may  be  easily,  but  only 
transitorily  effected.  The  magnet,  therefore,  attracts  it  powerfully,  convert- 
ing it,  however, .into  only  a  temporary  magnet.  In  steel,  the  two  kinds  of 
magnetism  are  nc-t  so  easily  separated ;  hence  the  latter  is  but  slightly  at- 
tracted by  the  most  powerful  magnets.  When  once  effected,  however,  the 
separation  is  permanent,  and  the  steel  becomes  a  perfect  magnet 

As,  according  to  this  theory,  the  act  of  rendering  a  body  magnetic  consists 
simply  in  decomposing  a  fluid  pervading  it,  we  can  easily  understand  how, 
by  means  of  one  artificial  magnet,  an  infinite  number  of  other  magnets  may 
be  made,  without  the  former  losing  any  of  its  magnetic  properties. 

what  is  a          796.    The  Magnetic  Needle  (Fig.  353)   is 
Magnetic  Nee-    simpiy  a  ^ar  of  steel,  which  is  a  magnet,  bal- 
anced upon  a  pivot  in  such  a  way  that  it  can 
turn  freely  in  a  horizontal  direction. 


424  WELLS'S  NATURAL  PHILOSOPHY. 

FlO.  353.  Such  a  needle,  when  properly  balanced,  will  bo 

observed  to  vibrate  more  or  less,  until  it  settles  in 
such  a  direction  that  one  of  its  extremities,  or 
poles,  points  toward  the  north,  and  the  other  to- 
ward the  south.  If  the  position  of  the  needle  bo 
altered  or  reversed,  it  will  always  turn  and  vibrato 
again  until  its  poles  havo  attained  the  same  direc- 
tion as  before. 

It  is  this  remarkable  property  of  a  magnetized 
steel  bar,  of  always  assuming  a  definite  direction, 

that  renders  the  compass  of  such  value  to  the  mariner,  the  engineer,  and  the 

traveler. 

what  is  a  The  ordinary  compass  consists  of  a  mag- 
compasB?  net-c  neecue)  or  kar  balanced  upon  a  pivot,  and 
inclosed  within  a  shallow  box,  or  metallic  case.  Upon 
the  bottom  of  the  box  is  a  circular  card  with  the  chief,  or 
cardinal  points  of  the  horizon,  north,  south,  east,  west, 
marked  upon  it. 

Fig.  354  represents  the  form  and  construction  of  the  ordinary,  or  land  com- 
pass. The  term  compass  is  derived  from  the  card,  "which  compares,  or  in- 
volves,  as  it  were,  the  whole  plane  of  the  horizon. 

FlG.  354. 


What  is  the  ^a  the  ^ea'  OT  ^ariner's  Compass,  the  needle  is  attached  to 
construction  of  the  under  side  of  the  card,  in  such  a  way  that  both  traverse 
Marines  Com-  together—the  needle  itself  being  out  of  sight.  Upon  the 
pass?  surface  of  the  card  is  engraved  a  radiating  diagram,  dividing 

the  whole  circle  of  the  horizon  into  thirty-two  parts,  called 
points.  The  compass-box  is  supported  by  means  of  two  concentric  hoops, 
called  gimbals.  These  are  so  placed  as  to  cross  each  other,  and  support  the 
box  immediately  in  the  center  of  the  two ;  so  that  whichever  way  the  vessel 


MAGNETISM. 


425 


may  roll  or  lurch,  the  card  is  al-  F\Q  355. 

ways  in  a  horizontal  position, 

and  is  certain  to  point  the  true 

direction  of  the  head  of  the  ship. 

Fig.  355  represents  the  construc- 

tion and  mounting  of  the  Sea 

Compass. 

w^is.D.p.       797.  If  a 

ping  Needle?         gimple       bar 

of  unmagnetized  steel, 

or  an   ordinary   needle 

be    suspended   from    a 

center,  instead  of  being  balanced  upon  a  pivot  beneath 

it,  it  will  hang  horizontally,  and  manifest  no  inclination 

to  dip  from  a  horizontal  line,  either 

on  one  side  or  the  other  of  the  cen- 

ter of  suspension.     But  if  the  bar, 

or  needle,  be  made  a  magnet,  it 

will  no  longer  lie  in  a  horizontal 

direction,  but  one  pole  will  incline 

downward  and  the  other  upward  ; 

the  inclination  in  this  latitude  to 

the  horizon  being  about  70°. 

Such  arrangement  is   called  a 
Dipping  Needle. 

Fig.  356.  represents  the  construction  and 
appearance  of  the  dipping  needle. 

798.     Although      the 

magnetic  needle  is  said 

to  point  north  and  south,  accurate  observations 
have  shown  that  it  does  not  point  exactly  north  and  south 
except  in  a  few  restricted  positions  upon  the  earth's  surface. 
what  is  the  799.  The  direction  assumed  bv  a  horizontal 

magnetic  mend-  . 

i»nf  needle  in  any  given  place  upon  the  earths 

surface,  is  called  the  magnetic  meridian. 

What  is  a  t  r  "^  terrestrial  meridian,  it  will  be  remembered,  is  a  great  cir- 
restriai  mend-  cle,  supposed  to  be  drawn  around  the  earth,  passing  through 
**nf  both  poles,  and  any  given  place  upon  its  surface,  and  inter- 

secting the  equator  at  right  angles.  (See  §  68,  Fig.  6,  page  36.)   The  direction 


Doe.then.ag- 


and  south. 


426  WELLS'S  NATUBAL  PHILOSOPHY. 

of  a  needle  which  would  point  due  north  and  south  at  anyplace,  will  bo 
the  true,  or  terrestrial  meridian  of  that  place. 

what  is  the        The  deviation  of  the  needle  from  the  true 
d^tto?'*    north  and  south,  or  the  angle  formed  by  the 
magnetic  meridian  and  the  terrestrial  merid- 
ian, is  called  the  variation,  or  declination  of  the  needle. 
__.  .  There  are  two  lines  upon  the  earth's  sur- 

What  arc  the  A 

ria£on?  no  Ta~    *ace?  a^ong  which  the  needle  does  not  vary,  but 
points  to  the  true  north  and  south.     These 
lines  are  called  the  eastern  and  western  lines  of  no  varia- 
tion, and  are  exceedingly  irregular  and  changeable. 

Their  position  is  as  follows:  —  The  western  line  of  no  variation  begins  in 
latitude  60°,  to  the  west  of  Hudson's  Bay,  passes  in  a  south  direction  through 
the  American  lakes,  to  the  West  Indies  and  the  extreme  eastern  point  of 
South  America.  The  eastern  line  of  no  variation  begins  on  the  north  in  the 
"White  Sea,  makes  a  great  semicircular  sweep  easterly,  until  it  reaches  the 
latitude  of  71°;  it  then  passes  along  the  Sea  of  Japan,  and  goes  westward 
across  China  and  Hindoostan  to  Bombay  ;  it  then  bends  east,  touches  Australia, 
and  goes  south. 

In  proceeding  in  either  direction,  east  or  west  from  the  lines  of  no  varia- 
tion, the  declination  of  the  needle  gradually  increases,  and  becomes  a  max- 
imum at  a  certain  intermediate  point  between  them.  On  the  west  of  the 
eastern  line  the  declination  is  west  ;  on  the  east  it  is  east. 

At  Boston,  in  the  United  States,  the  declination  of  the  needle  is  r.bout  5£° 
west  ;  in  England  it  is  about  24°  west  ;  in  Greenland,  50°  west  ;  at  St.  Peters- 
burg, 6°  west. 

.  800.  As  the  directive  property  of  the  magnetic  needle  is 

rective    power     observed  everywhere  in  all  parts  of  the  world,  on  all  seas,  on 


tbe  loftiest  summits  of  mountains,  and  in  the  deepest  mines, 
it  is  evident  that  there  must  be  a  magnetic  force  which  acts 
at  all  points  of  the  earth's  surface,  since  magnetic  needles  can  no  more  take 
up  a  direction  of  themselves  than  a  body  can  acquire  motion  of  itself.  To 
explain  these  phenomena,  the  earth  itself  is  considered  to  be  a  great  magnet, 
and  the  points  toward  which  the  magnetic  needle  constantly  turns  are  called 
the  magnetic  poles  of  the  earth.  These  poles,  by  reason  of  their  attractive 
influence,  give  to  the  needle  its  directive  power. 

The  two  poles  of  the  great  terrestrial  magnet  which  are 
magnetic  poles  situated  in  the  vicinity  of  the  poles  of  the  earth's  axis,  are 
situated  ?  termed  respectively  the  magnetic  north  pole  and  the  magnetic 

south  pole.     These  contrary  poles  attract  each  other,  and  thus  a  magnetic 
needle  will  turn  its  south  pole  to  the  north,  and  its  north  pole  to  the  south. 
Hence,  what  we  generally  call  the  north  pole  of  a  needle  is  in  reality  its 
eouth  pole,  and  its  south  pole  is  its  north  pole. 
The  exact  position  of  the  northern  magnetic  pole  is  about  19°  from  the  north 


MAGNETISM. 


427 


If  a  compass 
needle  be  car- 
ried to  the  mag- 
netic pole  what 
will  occur  ? 


dipping  needle 
vary? 


pole  of  the  earth,  in  the  direction  of  Hudson's  Bay.  It  was  visited  by  Sir  J. 
Ross  in  1832,  in  his  voyage  of  Arctic  discovery.  The  south  magnetic  pole  is 
situated  in  the  antarctic  continent,  and  has  been  approached  within  170  miles. 

If  the  ordinary  compass  be  carried  to  either 
of  the  magnetic  poles,  it  will  lose  its  power 
and  point  indifferently  in  any  direction.  If  it 
is  carried  beyond  the  magnetic  pole,  to  any 
point  between  it  and  the  true  pole,  the  poles  of  the  needle 
become  reversed,  the  end  called  the  north  pole  pointing  to 
the  south,  and  the  south  to  the  north. 

How  does  the  The  Position  assumed  by  the  dipping  needle  varies  in  dif- 
position  of  the  ferent  latitudes.  If  it  were  carried  directly  to  the  north  mag- 
netic pole,  its  south  pole  would  be  attracted  downward,  and 
the  needle  would  stand  perfectly  upright.  At  the  south  mag- 
netic pole,  its  position  would  be  exactly  reversed.*  If  the  dipping  needle  be 
taken  to  the  equator  of  the  earth,  or  to  a  point  midway  between  the  north 
and  south  magnetic  poles,  it  will  be  attracted  equally  by  both,  and  will  re- 
main perfectly  horizontal,  or  cease  to  dip  at 
all :  as  we  go  north  or  south,  however,  it  dips 
more  and  more,  until  at  the  magnetic  poles, 
as  before  stated,  it  becomes  perpendicular—- 
the end  which  was  uppermost  at  the  north 
being  the  lowest  at  the  south.f 

Fig.  357  represents  the  position  assumed 
|  by  the  magnetic  njeedle  in  various  latitudes. 
The  magnetic  poles  of  the  earth  are  not 
stationary,  but  change  their  position  grad- 
ually during  long  intervals  of  time. 

Observations  on  the  temperature  of  the 
earth  have  afforded  some  reason  for  believ- 

*  Like  the  declination  and  dip,  the  intensity  of  the  earth's  magnetism  varies 
very  much  in  different  parts  of  the  earth  ;  at  the  magnetic  equator  being  the  most  feeble, 
and  gradually  increasing  as  we  approach  the  poles.  The  intensity  of  terrestrial  magnetism 
in  different  places  may  be  measured  by  the  number  of  vibrations  made  by  a  magnetic 
needle  in  a  given  time. 

t  As  the  directive  tendency  of  the  horizontal  needle  arises  from  its  poles  being  attracted 
by  those  of  the  earth,  it  is  evident  from  the  rotundity  of  the  earth,  that  its  poles  will  not 
be  attracted  by  those  of  the  earth  horizontally,  but  downward,  so  that  the  needle  can  not 
tend  to  be  horizontal,  except  when  it  i«  acted  upon  by  both  poles  equally— that  is,  when 
midway  between  them.  When  nearer  the  north  magnetic  pole  than  the  south,  its  north 
end  must  be  attracted  downward,  and  the  contrary  when  it  is  nearest  the  south  pole. 
Accordingly,  a  needle  which  was  accurately  balanced  on  its  support  before  being  mag- 
netized,  will  no  longer  balance  itself  when  magnetized,  but  in  this  country  its  north  pole 
will  appear  to  dip,  or  appear  to  be  the  heavier  end.  This  circumstance  has  to  be  corrected 
in  ships'  compasses  by  a  small  sliding  weight  attached  to  the  southern  half,  which  weight 
has  to  be  removed  on  approaching  the  equator,  and  shifted  to  the  other  side  of  the  needle 
when  in  the  northern  hemisphere. 


428  WELLS'S  NATURAL  PHILOSOPHY, 

ing,  that  the  points  upon  the  earth's  surface  where  the  greatest  degree  of  cold 
is  experienced,  or  where  the  yearly  mean  of  the  thermometer  is  lowest, 
coincides  with  the  location  of  the  magnetic  poles. 

what  is  the  801.  Beside  the  variation  from  the  true 
«ou™alofvarthe  north  and  south,  the  magnetic  needle  is  sub- 
needie?  jec£  ^o  a  diurnal  variation.  This  movement, 

or  variation,  commences  about  seven  in  the  morning,  when 
the  north  end  of  the  needle  begins  to  deviate  toward  the 
west ;  it  reaches  its  maximum  deviation  about  two  o'clock 
in  the  afternoon,  when  it  begins  to  return  slowly  to  its 
original  position. 

The  magnetic  needle  is  subject  also  to  an  annual  movement,  and  a  move- 
ment different  in  the  winter  months  from  that  noticed  in  the  summer  months. 

what  is  the  ^he  daily,  monthly,  and  yearly  variations 
o?  the JerfodiSi  °^  tae  needle  are  supposed  to  be  occasioned  by 
thl^eedie  ?  °f  variations  in  the  temperature  of  the  earth's 
surface,  depending  upon  the  changes  in  the 
position  and  action  of  the  sun. 

Observations  made  for  a  great  number  of  years  seem  to  show  that  the  en- 
tire magnetic  condition  of  the  earth  is  subject  to  a  periodical  change,  but 
neither  the  cause  or  the  laws  of  this  change  are  as  yet  understood. 

For  most  practical  operations,  as  in  navigation  and  sur- 
veying, the  deviation  of  the  magnetic  needle  from  the  true 
north  and  south,  is  carefully  taken  into  account,  and  a  rule 
of  corrections  applied.  A  knowledge  of  the  amount  of  vari- 
ation, east  or  west,  for  different  localities  upon  the  earth's 
surface,  may  be  obtained  from  tables  accurately  arranged 
for  this  purpose. 

The  variation  of  the  magnetic  needle  from  the  true  north  and  south,  is  said 
to  have  been  first  noticed  by  Columbus  in  his  first  voyage  of  discovery.  It 
was  also  observed  by  his  sailors,  who  were  alarmed  at  the  fact,  and  urged  it 
as  a  reason  why  he  should  turn  back. 

when  was  the  ^ ne  compass  is  claimed  to  have  been  dis- 
cSJX8?  dis"  covered  by  the  Chinese :  it  was,  however, 
known  in  Europe,  and  used  in  the  Mediterra- 
nean, in  the  thirteenth  century.  The  compasses  of  that 
time  were  merely  pieces  of  loadstone  fixed  to  a  cork,  which 
floated  on  the  surface  of  water. 

802.  The  resemblance  between  magnetism  and  electricity  is  very  striking, 
and  there  axe  good  reasons  for  believing  that  both  are  but  modifications  of 


ELECTRO-MAGNETISM.  429 

one  force.  Both  are  supposed  to  consist  of  two  fluids,  which  repel  their  own 
kind,  and  attract  the  opposite.  The  fluid  in  both  cases  is  supposed  to  reside 
upon  the  surface  of  bodies;  the  laws  of  induction  in  both  are  the  same;  and 
each  can  be  made  to  excite  or  develop  the  other. 


CHAPTER    XIX. 

ELECTRO-MAGNETISM. 

what  is  Eiee-  80S.  MAGNETISM  developed  through  the 
tro-inagnetism?  agenCy  of  electrical  or  chemical  action,  is 
termed  Electro-magnetism. 

Among  the  earliest  phenomena  observed  which  indicated  a  connection  be- 
tween magnetism  and  electricity,  it  was  noticed  that  ships'  compasses  have 
their  directive  power  impaired  by  lightning,  and  that  sewing  needles  are  ren- 
dered magnetic  by  electric  discharges  passed  through  them. 

.  In  1820,  a  discovery  was  made  by  Professor  Oersted  of 
produced  when  Denmark,  which  established  beyond  a  doubt  the  connection 
diTi^brou"  ht  of  electricitv  an(i  magnetism.  He  ascertained  that  a  mag- 
near  a  conduct-  netic  needle  brought  near  to  a  wire,  through  which  an  electric 
current  was  circulating,  was  compelled  to  change  its  natural 
direction,  and  that  the  new  direction  it  assumed  was  determined  by  its  position  in 
relation  to  the  wire  and  to  the  direction  of  the  current  transmitted  along  the  wire. 

Further  experiments  developed  the  following  law : — 

in  what  direc-  Electric  currents  exert  a  magnetic  influence 
cu"rentsleexS  at  rignfc  angles  with  the  direction  of  their  flow, 

their  influence?      aQ(J    wnen    tney    act    UpOQ    ft    magnetiC    needle 

FIG.  358.  they  tend  to  cause  the  needle 

33 ^       — -A  to  assume  a  position  at  right 

angles  to  the  direction  of  the 
_:>  S     current. 

Thus,  suppose  an  electric  current  to 
pass  on  the  wire  A  B,  Fig.  358,  in  tho 
direction  of  the  arrow ;  suppose  a  mag- 
netic needle,  N  S,  to  be  placed  directly 
under  the  wire  and  parallel  to  it.  By 
the  action  of  the  electric  current  flowing 
in  the  direction  A  B,  the  needle  is  caused 
to  move  from  its  north  and  south  posi- 
tion and  turn  round,  and  if  the  current 


430 


WELLS'S  NATURAL  PHILOSOPHY. 


PlG.  359. 


is  sufficiently  strong,  it  will  place  itself  at  right  angles  with  the  wire,  as  is 

represented  in  the  figure. 

If  the  current,  however,  had  passed  in  the  same  direction  below  the  needle, 

instead  of  above  it  as  in  the  first  instance,  the  deflection  ot  the  needle  would 

have  taken  place  as  before,  but  in  an  opposite  direction,  the  pole  S  standing 

where  the  pole  N  did  previously,  and  N  also  in  the  place  of  S. 

In  like  manner,  if  the  needle  be  placed  by  the  side  of  the  wire,  a  like  effect 

will  be  produced;  on  one  side  it  dips  down,  and  on  the  other  it  rises  up;  and 
in  whatever  other  position  the  needle  may  be 
placed,  it  will  always  tend  to  set  itself  at  right 
angles  to  the  current.  If  the  wire  be  bent  in 
the  form  of  a  rectangle,  as  is  represented  in  Fig. 
359,  so  as  to  carry  the  current  around  tho 
needle,  above  and  below  it.  in  opposite  direc- 
tions, the  opposite  currents,  instead  of  neu- 
tralizing, will  assist  each  other,  and  the  needle 

will  move  in  accordance  with  the  first  direction  of  the  current. 

If  the  wire,  instead  of  making  a  single  turn,  is  bent  many  times  nroundthe 

needle,  the  magnetic  force  excited  by  the  current  of  electricity  traversing  the 

wire,  will  be  greatly  increased,  the  increase  being,  within  certain  limits,  pro- 
portional to  the  number  of  turns  of  the  wire. 

It  is  upon  this  principle  that  an  instrument  called  tho  Gal- 

Galv"nometer?     vanometer,  for  measuring  the  quantity  of  an  electric  current, 
is  constructed.    It  consists  of  a  rectangular  coil  of  copper 
FlG.  360.  wire,  N  B  S,  Fig.  3GO,   containing  about  20 

convolutions,  the  separate  coils  being  insulat- 
ed by  winding  the  wire  with  silk  thread.  A 
magnetic  needle,  supported  on  a  pivot,  is 
placed  in  the  center  of  the  coil,  and  a  gradu- 
ated circle  is  fixed  below  it  to  measure  the 

amount  of  the  deflection  ;   the  two  ends  of  the  wire  connect  with  two  cups, 

C  and  Z,  which  contain  mercury,  and  into  which  the  poles  of  the  battery 

transmitting  the  current  dip. 

In  this  form  of  the  instrument 
the  transmitted  current  is  obliged 
to  contend  with  the  influence  of 

the  earth's  magnetism,  which  tends  to  hold  tho 

needle  in  its  original  position,  and  unless  the 

former  is  more  powerful  than  the  latter,  the 

needle  is  not  moved.     This  difficulty  has  been 

overcome  by  means  of  an  arrangement  called 

the  Astatic  Needle.    This  consists  essentially  of 

two  needles  fastened  together,  one  above  the 

other,  but  with  their  poles  in   opposite  direc- 
tions, as  is  represented  in  Fig.  361.    In  this 

way  the  influence  of  the  earth  is  almost  entirely 

emov»d,  and  the  force  of  the  transmitted  current  is  rendered  more  effective. 


What  is  the  As- 
tatic  Needle? 


FIG.  361. 


ELECTRO-MAGNETISM. 


431 


In  what  man- 
ner does  an 
electric  current 
exert  its  mag- 
netic force  ? 


FlG.  362. 


By  means  of  the  galvanometer,  the  most  feeble  traces  of  electricity  can  be 
detected ;  and  electric  currents  which  would  fail  to  influence  the  most  sensi- 
tive gold  leaf  electrometef  can  be  made  to  affect  perceptibly  the  magnetic 
needle.  Galvanometers  are  sometimes  called  electro-multipliers. 

804.  Electricity,  unlike  all  other  motive 
forces  in  nature,  exerts  its  magnetic  force  lat- 
erally ;  all  other  forces  exerted  between  two 
points  act  in  the  direction  of  a  straight  line 
connecting  their  points,  but  the  electric  current  exerts  its 
magnetic  influence  at  right  angles  to  the  direction  of  its 
course. 

When  a  magnetic  pole  is  influenced  by  an  electric  cur- 
rent, it  does  not  mote  either  directly  toward  or  directly 
from  the  conducting  wire,  but  it  tends  to  revolve  about  it. 

By  the  application  of  these  facts,  it  has  been  discovered  that  rotatory  move- 
ments can  be  produced  by  magnets  around  conducting  wires,  and  conversely, 
that  conducting  wires  can  be  made  to  rotate  around  magnets. 

The  rotation  of  the  pole  of  a  magnet  around  a  fixed  conducting 
wire  may  be  shown  by  a  piece  of  apparatus  represented  by  Fig. 
362.  A  small  magnet,  N,  is  fixed  to  the  lower  part  of  a  vessel, 
V,  by  means  of  a  silk  thread ;  the  vessel  is  filled  with  mercury 
nearly  to  the  top  of  the  magnet;  G-  is  a  conducting  wire  dipping  into 
the  mercury,  and  Z  is  another  conductor  communicating  with  the 
mercury  at  the  bottom  of  the  vessel.  Now,  when  the  electric 
current  is  established,  by  connecting  the  extremities  of  the  wires 
C  and  Z  with  the  opposite  poles  of  the  battery,  the  pole  N  of  the 
magnet  revolves  round  the  conducting  wire  C.  If  the  current  is 
descending,  that  is,  if  C  be  connected  with  the  positive  pole  of 
the  battery,  and  if  N  be  a  north  pole,  its  motion  round  the  wire  will  be  di- 
rect, that  is.  in  the  direction  of  the  hands  of  a  watch;  and 
so  on,  vice  versa. 

A  different  arrangement,  by  which  a  movable  wire  tra- 
versed by  a  current,  may  be  made  to  revolve  around  the 
pole  of  a  fixed  magnet,  is  represented  by  Fig.  363.  A  wire, 
A  B,  is  suspended  from  the  wire  C  by  a  loop,  and  dips  into 
the  mercury  in  a  vessel,  V ;  when  the  circuit  is  established, 
by  connecting  C  and  N  with  the  respective  poles  of  the 
battery,  the  conducting  wiro  revolves  around  the  pole  N 
of  the  magnet. 

If  the  current  be  descending,  and  N  be  the  north  pole  of 
the  magnet,  the  rotation  will  be  direct. 

On  similar  principles,  various  kinds  of  reciprocating  and  rotatory  movements 
may  be  produced. 


432 


WELLS'S   NATURAL   PHILOSOPHY. 


What  is  an 
Electro-mag- 
net? 

What    ia     a 
Helix? 


in  what  man-  805.  If  o>  piece  of  soft  iron,  entirely  wanting 
eiectricacurren"  in  magnetism,  be  placed  within  a  coil  of  wire 
cftema magnet  through  which  an  electric  current  is  circulat- 
ing, it  will  be  rendered  intensely  magnetic,  so 
long  as  the  current  continues  ;  but  the  moment  the  cur- 
rent ceases,  the  iron  loses  its  magnetism. 

Magnets  formed  in  this  way,  through  the 
agency  of  electricity,  are  called  Electro-mag- 
nets, and  are  more  powerful  than  any  others. 
The  coil,  or  spiral  line  of  wire  used  for  excit- 
ing magnetism  in  the  iron  by  conducting  a 
current  of  electricity  about  it,  is  called  a  Helix. 

FIG  364  ^  *s  usua^7  made  of  copper  wire,  coated  with 

some  non-conducting  substance,  such  as  silk  wouiid 
round  it.  The  coils  of  the  "wire  are  generally  re- 
peated one  over  the  other,  until  the  size  of  the  helix 
is  sufficient,  since  the  magnetic  action  of  an  electric 
current  upon  a  bar  of  iron  increases  to  a  certain  ex- 
tent with  the  number  of  revolutions  it  performs  about 
it  Fig.  364  represents  the  appearance  of  a  helix. 

It  is  necessary  for  the  induction  of  magnetism  in  iron 
bars  by  electricity,  that  the  current  should  flow  at  right 
angles  to  the  axis  of  the  bars. 

1^  the  ^ar  ^e  steel,  the  magnetism  thus  m- 
duced  will  be  permanent ;  and  the  direction  in 
magnet?  which  the  current  moves  round  the  helix  de- 

termines which  of  its  extremities  shall  constitute  its  north, 
and  which  its  south  pole. 

"When  the  current  circulates  in  the  direction  of  the  hands 
of  a  watch,  the  north  polo  of  the  bar  will  be  at  the  farthest 
end  of  the  helix. 

If  a  bar  of  soft  iron,  bent  in  the  form  of  a  horse  shoe  mag- 
net, be  wound  with  insulated  wire,  as  is  represented  in  Fig. 
365,  and  a  current  of  electricity  transmitted  through  it,  it 
becomes  a  most  powerful  magnet. 

Electro-magnets  of  this  character  have  been  formed  capa- 
ble of  supporting  more  than  a  ton  weight.  The  magnetic 
power  thus  developed  is  wholly  dependent  upon  the  ex- 
istence of  the  current,  and  the  moment  it  ceases  the  weights 
fall  away  by  the  action  of  gravity. 


What      deter- 


FIG.  365. 


ELECTRO-MAGNETISM. 


433 


FiG.  366. 


been     applied 

to  any  prac- 
tical purpose 
for  propelling 
machinery  ? 


If  two  semicircular  rings  of  soft  iron  be  passed  within  a 
helical  ring,  as  is- represented  in  Fig.  366,  they  will  become  so 
strongly  magnetic  on  passing  the  current  of  even  a  small 
battery,  as  to  be  separated  with  extreme  difficulty.  A  rod 
of  iron  brought  near  to  one  of  the  extremities  of  a  longitudinal 
helix,  is  not  only  attracted  but  lifted  up  into  the  center  of  the 
coil,  where  it  remains  suspended  without  contact  or  visible 
support,  so  long  as  the  current  continues  in  action.  If  the 
battery  and  helix  be  of  sufficient  size,  a  considerable  weight 
may  be  suspended.  In  some  experiments  at  the  Smithsonian 
Institution  at  Washington,  a  few  years  since,  a  bar  of  iron 
weighing  80  pounds  was  raised  and  suspended  in  the  air  with- 
out being  in  contact  with  any  body. 

806.  Many  attempts   have   been  made  to 

Has      electro-       ,    -i  i  ,  r>    ,  ,  /. 

magnetic  force  take  advantage  of  the  enormous  force  gener- 
ated and  destroyed,  in  an  instant,  by  making 
or  breaking  an  electric  current,  for  propelling 
machinery,  but  thus  far  all  efforts  have  failed 

to  produce  any  practical  results. 

One  of  the  reasons  why  this  power  can  not  be  used  to  advantage  is,  that 
the  rate  at  which  the  power  diminishes  as  we  recede  from  the  contact  point 
of  the  magnets,  prevents  our  obtaining  the  full  force  of  the  magnets.  Thus, 
a  magnet  whose  force  in  contact  would  be  sufficient  to  raise  250  pounds, 
would  exert  a  force  of  only  90  pounds  at  the  distance  of  1-2 50th  of  an  inch, 
and  of  only  40  pounds  at  the  distance  of  l-50th  of  an  inch.  It  is  also  found 
that  notwithstanding  the  loss  of  power  with  distance,  a  still  greater  loss  takes 
place  with  motion.  The  moment  any  magnetic  body  is  moved  in  front  of 
either  a  permanent  or  an  electro-magnet,  it  loses  power,  and  this  loss  increases 
very  rapidly  with  the  increase  of  velocity.  This  obstacle  stopped  the  prog- 
ress of  the  very  extensive  researches  of  Professor  Jacobi,  after  he  had  ex- 
pended upward  of  $120,000  granted  him  for  his  experiments  by  the  liber- 
ality of  the  Russian  government. 

807.  The  construction  of  the  Morse  mag- 
netic telegraph  depends  upon  the  principle, 
that  a  current  of  electricity  circulating  about 
a  bar  of  soft  iron   temporarily  renders  it  a 

magnet. 

The  construction  and  method  of  operating  the  Morse  telegraph  may  bo 
clearly  understood  by  reference  to  Fig.  367.  F  and  B  are  pieces  of  soft  iron 
surrounded  by  coils  of  wire,  which  are  connected  at  a  and  &  with  wires  pro- 
ceeding from  a  galvanic  battery.  When  a  current  is  transmitted  from  a  bat- 
tery located  one,  two,  or  three  hundred  miles  distant,  as  the  case  may  be,  it 

in 


Upon       what 


Morse       tele- 
graph depend? 


434 


WELLS'S   NATURAL   PHILOSOPHY. 


passes  along  the  wires,  and  through  the  coils*  surrounding  tho  pieces  of  soft 
iron,  F  and  E,  thereby  converting  them  into  magnets.  Above  these  pieces 
of  soft  iron  is  a  metallic  bar,  or  lever,  A,  supported  in  its  center,  and  having 
at  one  end  the  arm,  D,  and  at  the  other  a  small  steel  point,  o.  A  ribbon  of 
paper,  p  ht  rolled  on  the  cylinder,  B,  is  drawn  slowly  and  steadily  off  by  a 
train  of  clock-work,  K,  moved  by  the  action  of  the  weight,  P,  on  the  cord,  C. 
This  clock-work  gives  motion  to  two  metal  rollers,  G  and  H,  between  which 
the  ribbon  of  paper  passes,  and  which,  turning  in  opposite  directions,  draw 
the  paper  from  the  cylinder  B.  The  roller  H  has  a  groove  around  its  circum- 
ference (not  represented  in  the  engraving),  above  which  the  paper  passes. 
The  steel  point  o  of  the  lever  A  is  also  directly  opposite  this  groove.  The 
spring,  r,  prevents  the  point  from  resting  upon  the  paper  when  the  telegraph 
is  not  in  operation. 

FIG.  367. 


The  manner  in  which  intelligence  is  communicated  by  these  arrangements 
is  as  follows :  The  pieces  of  soft  iron,  F  and  E,  being  rendered  magnetic  by 
the  passage  of  a  current  of  electricity  transmitted  from  the  battery  through  the 
coils  of  wire  surrounding  them,  attract  the  metal  arm  D  of  the  lever  A.  The 
end  of  the  lever  at  D  being  depressed,  the  steel  point  o  at  the  other  extremity 
is  elevated  and  caused  to  press  against  the  paper  ribbon  and  indent  it.  When 
the  current  from  the  battery  is  broken  or  interrupted,  the  pieces  of  soft  iron 
F  and  E  being  no  longer  magnetic,  cease  to  attract  the  arm  D.  The  lever 
A  is  therefore  drawn  back  to  its  former  position  by  the  action  of  the  spring  r, 
and  the  steel  point  o  ceases  to  indent  the  paper.  By  letting  the  current  flow 

•  These  coils  consist  of  very  fine  copper  wire,  some  thousands  of  feet  being  gener- 
ally contained  in  them.  In  this  way  a  magnet  of  small  size  and  great  power  may  be 
obtained. 


ELECTRO-MAGNETISM. 


435 


How  many 
wires  are  nec- 
essary for 
working  the 
telegraph? 


FIG.  368. 


round  the  magnet  for  a  longer  or  shorter  time  a  dot,  or  a  line  is  made,  and 
the  telegraphic  alphabet  consists  of  a  series  of  such  marks.* 

Grove's  battery  (see  Fig.  340)  is  generally  used  for  working  the  telegraph, 
about  thirty  cups  being  required  for  a  distance  of  150  miles.  These  cups 
may  be  kept  in  one  compact  space,  but  operate  the  telegraph  more  success- 
fully when  distributed  along  the  line.  Such  batteries  will  work  for  about  two 
weeks  without  replenishing. 

Formerly  two  wires  were  required  in  telegraphing;  'one 
conveyed  the  current  from  the  battery  to  the  electro-magnet, 
at  a  distance,  through  which  it  passed,  and  then  returned  by 
another  wire  back  to  the  battery.     At  present  but  one  wire  ia 
generally  used.     It  was  found  that  the  earth  itself  might  be 
made  to  perform  the  function  of  the  returning  wire.     To  effect  this  all  that  is 
necessary  is  that  one  short  wire  from  the  battery  at  one  end  of  a  line,  and 
from  the  electro-magnet  at  the  other  end,  should  be  sunk  into  the  moist 

earth,  and  there  connected  with  a 
mass  of  conducting  metal,  from 
which  the  electricity  passes  to 
complete  the  closed  circuit 

For  interrupting  the 
current  and  regulating 
the  system  of  dots  and 
lines,  an  instrument  call- 
ed the  Signal-key,  or, 
Break-piece,  Fig.  368, 
is  employed.  This  is 
placed  near  the  battery,  so  as  to  be  in  the  galvanic  cir- 
cuit. The  operator,  by  pressing  down  the  knob  with  the 
finger,  closes  the  circuit  and  allows  the  current  to  pass,  but 
when  the  pressure  is  removed  communication  is  interrupted 

*  The  following  table  exhibits  the  signs  employed  to  represent  letters  in  the  Morse 


NUMERALS. 

1 

2  _ 


system  of  telegraphing: 

ALPHABET, 
a  
b  

n  
o-  - 

c  —    - 
d-:  
«- 
/  

ill"" 

1  

m  

r  -    — 
*  
t  

w  

<-  

V--    -- 
z  
<fe  _    

Experienced  operators  are  often  able  to  understand  the  message  merely  from  the  sounds, 
or  clicks,  of  the  lever. 


436  WELLSrS   NATURAL   PHILOSOPHY. 

wimt  is  the  808.  In  what  is  known  as  the  "  Bain,"  or 
?hestrc£micai  chemical  telegraph,  there  is  no  magnet  created, 
telegraph?  ^  a  small  gteel  W'ITG}  connected  with  the 

wire  from  the  line,  presses  upon  a  roll  of  paper,  moved 
by  clock-work.  This  paper,  before  being  coiled  on  the 
roller,  has  been  dipped  in  a  nearly  colorless  chemical  solu- 
tion, which  becomes  colored  when  an  electric  current  passes 
through  it.  By  sending  a  current  through  the  wire  rest- 
ing on  the  paper,  we  can  stain  it,  as  it  were,  in  dots  and 
lines  in  the  same  manner  as  the  last  instrument  em- 
bossed it  in  dots  and  lines. 

809.    The  House's,   or  printing  telegraph, 

What    is    the       ,.™          «  ,  '  K      .       ,?      . 

tele-    diners  from   the  others  principally  in  an  ar- 


rangement whereby  the  message  as  transmitted 
is  printed  in  ordinary  letters,  at  the  rate  of  two  or  three 
hundred  a  minute. 

what  was  the  810..  The  method  first  proposed  for  com- 
mShod^pro-  niunicating  intelligence  by  electricity  was  by 
posed?  deflecting  a  compass  needle  by  causing  a  cur- 

rent to^pass  along  its  length. 

Thus,  if  at  a  given  point  we  place  a  galvanic  battery,  and  at  a  hundred 
miles  from  it  there  is  fixed  a  compass  needle,  between  a  wire  brought  from, 
and  another  returning  to  the  battery,  the  needle  will  remain  true  to  its  polar 
direction  so  long  as  the  wires  are  free  from  the  excited  battery  ;  but  the  mo- 
ment connection  is  made,  the  needle  is  thrown  at  right  angles  to  the  direc- 
tion of  the  current.  The  motion  of  the  needle  may  thus  be  made  to  convey 
intelligence. 

It  is  necessary,  in  conveying  the  wires  from  point  to  point,  to  support  them 
on  the  poles  by  glass  or  earthen  cj-linders,  in  order  to  insure  insulation, 
otherwise  the  electricity  would  pass  down  a  damp  pole  to  the  earth,  and  be 

lost. 

811.  The  idea  that  many  persons  have,  that  some  substance 
Spie  '™7influ-  passes  along  the  telegraphic  wires  when  intelligence  is  trans- 
ence  pass  along  raitted,  is  wholly  erroneous  ;  the  word  current,  as  something 
a  emessagc  IS  flowing,  expresses  a  false  idea,  but  we  have  no  other  term  to 
communicated?  express  electrical  progression.  "We  may,  however,  gain  some 
idea  of  what  really  takes  place,  and  of  the  nature  of  the  influence  transmitted, 
by  remembering  that  the  earth  and  all  substances  are  reservoirs  of  electricity  ; 
and  if  we  disturb  this  electricity  at  any  given  point,  as  at  Washington,  its  pulsa- 
tions may  be  felt  at  New  York.  Suppose  the  telegraphic  wire  a  tube  extend- 
ing from  "Washington  to  New  York  perfectly  filled  with  water;  now,  if  one 


ELECTRO-MAGNETISM.  4371 

drop  more  is  forced  into  the  tube  at  Washington,  a  drop  must  fall  out  at 
New  York,  but  no  drop  is  caused  to  pass  from  Washington  to  New  York. 
Something  like  this  occurs  in  the  transmission  of  electricity. 

812.  Electricity,   through   an   electro-mas;- 

Can  electricity 

be    made   to    netic  arrangement,  can  be  made  available  for 

measure  time?  „     .  i   i        • 

the  measurement  of  time,  and  by  its  agency  a 
great  number  of  clocks  can  be  kept  in  a  state  of  uniform 
correctness. 

The  plan  by  which  this  is  accomplished  is  substantially  as  follows: — A  bat- 
tery being  connected  with  a  principal  clock,  which  is  itself  connected  by 
means  of  wires  with  any  number  of  clocks  arranged  at  a  distance  from  each 
other,  has  the  current  regularly  and  continually  broken  by  the  beating  of  the 
pendulum.  This  interruption  is  also  experienced  by  all  the  clocks  included 
in  the  circuit;  and  in  accordance  with  this  breaking  and  making  of  contact, 
the  indicators  or  hands  of  the  clock  move  over  the  dial  at  a  constantly  uniform 
rate. 

813.  The  fundamental  law  of  action  in  frictional  electricity 
Son  of8  ciel  isi  that  bodies  charged  with  like  electricities  at  rest  repel,  and 
trical  currents  with  unlike,  attract  each  other.  With  electricity  in  motion 
upoi  each  '  ,  .  ,  ,  j./v.  .  c  xi 

other?  the  case  is  somewhat  different,  since  currents  of  the  same 

electricity  moving  in  the  same  direction  attract  each  other. 
The  general  law  of  this  action  may  be  stated  as  follows : 

what  is  the  ^  Q^QC^T'1C  currents  flow  in  wires  parallel  to 
fhKctiS  °f  eac^  otner?  an(i  have  freedom  of  motion,. the 
wires  are  immediately  disturbed.  If  the  cur- 
rents are  moving  in  the  same  direction,  the  wires  attract 
each  other ;  if  they  are  moving  in  opposite  directions, 
they  repel  each  other :  or,  like  currents  attract,  and  un- 
like repel. 

iiowmayahe-  814.  When  the  wires  connecting  the  positive 
i"  inatoTmenr£  au&  negative  poles  of  a  galvanic  battery  in  ac- 

netic  needle  f       tjon  are  ^faft  JQ  ^  form  Qf  ft  ^j^  ^  ^^ 

becomes  possessed  of  magnetic  properties.  If  such  a 
helix  be  suspended  in  a  horizontal  plane,  it  points,  as  a 
magnetic  needle  would,  north  and  south  ;  if  it  is  sus- 
pended so  as  to  move  in  a  vertical  plane,  it  acts  as  a  dip- 
ping needle. 

If  two  helices  carrying  currents  are  presented  to  each  other,  they  attract 
and  repel,  precisely  as  if  they  were  magnets,  according  as  like  or  unlike  poles 
are  brought  together.  And,  in  short,  all  the  properties  of  the  magnetic  needle 
may  be  imitated  by  a  helix  carrying  a  current. 


438 


WELLS'S   NATURAL   PHILOSOPHY. 


What  is  Aim 
pere's  theory 
of  magnetism? 


815.  From  these,  and  other  like  phenomena,  M.  Ampere 
has  propounded  a  theory  which  accounts  for  nearly  all  the 
phenomena  of  terrestrial  magnetism. 
He  supposes  that  all  magnetic  phenomena  are  the  result  of  the  circulation 
of  electrical  currents.  Every  molecule  of  a  magnet  is  considered  to  be  sur- 
rounded with  an  atmosphere  of  electricity,  which  is  constantly  circulating 
around  it,  the  difference  between  a  magnet  and  a  mere  bar  of  iron  being,  that 
the  electricity  which  exists  equally  in  the  iron,  is  at  rest,  whereas  in  the  mag- 
net it  is  in  motion.  The  direction  of  these  currents  circulating  in  a  magnet 
is  dependent  upon  the  position  in  which  the  magnet  is  held.  If  the  opposite 
or  unlike  poles  of  two  magnets  be  placed  end  to  end,  the  electric  currents  of 
each  will  be  found  running  the  same  way,  and  as  currents  moving  in  the  same 
direction  attract  each  other,  the  two  poles  will  tend  to  come  together.  On 
the  contrary,  if  the  ends  of  like  poles  be  presented,  the  course  of  the  currents 
traversing  each  will  be  in  opposite  directions,  and  a  repulsion  will  result. 

A  magnetic  needle  tends  to  arrange  itself 
at  right  angles  with  a  wire  transmitting  an 
electric  current,  in  order  to  bring  the  numer- 
ous currents  circulating  around  its  particles 
parallel  with  that  of  the  wire. 

The  magnetism  of  the  earth  is  also  explained  by  this  theory 
on  the  same  principles.  If  we  take  a  metal  ring  and  warm 
it  at  one  point  only  by  a  spirit-lamp,  no  electrical  effect  en- 
sues ;  but  if  the  lamp  is  moved  an  electric  current  runs  round 
the  ring  in  the  direction  the  lamp  has  taken.  In  a  like  man- 
ner, currents  of  electricity  are  known  to  be  excited  and  kept  in  motion  around 
the  earth,  by  the  sun,  which  heats  in  turn  successive  portions  of  its  surface. 
They  flow  round  it  from  east  to  west  in  a  direction  at  right  angles  with  a  line 
joining  the  magnetic  poles.  A  magnetic  needle,  therefore,  points  north  and 
south,  because  that  position  is  the  one  in  which  the  electric  currents  in  it  are 


Why  does  a 
magnetic  nee- 
dle tend  to  ar- 
range 


a  current? 


How  is  the 
magnetism  of 
the  earth  ex- 
plained by  this 
theory  ? 


FiG.  369. 


parallel  to  those  of  the  earth,  and 
this  is  the  position,  as  has  just 
been  explained,  that  electric  cur- 
rents tend  always  to  assume. 

Fig.  369  represents  an  artificial 
globe,  surrounded  by  a  coil  of  in- 
sulated wire,  surmounted  by  a 
magnetic  needle.  The  needle  will 
always  point  to  the  north  pole  of 
the  globe,  on  transmitting  the  bat- 
tery current. 

The  dip  of  the  needle  may  be 
also  readily  accounted  for  in  the 
same  manner.  At  the  polar  re- 


ELECTRO-MAGNETISM. 


439 


FIG.  370. 


gions  it  dips  vertically  down  in  order  that  its  currents  may  be  parallel  with 
those  of  the  earth  ;  for  in  those  regions  the  sun  performs  his  daily  motion  in 
circles  parallel  to  the  horizon.  At  the  equator,  the  course  of  the  sun  is 
nearly  at  right  angles  to  the  horizon,  and  the  needle  maintains  a  horizontal 
position. 

what  is  Mag-        816.  As  an  electric  current   passing  round 

neto-electricity  ?    ^  exj;erior  Qf  a  |jar  Qf   soft  jron  in(Juces  mag- 

netism  in  it,  so  on  the  contrary,  a  magnetized  bar  is  able 
to  generate  an  electric  current  in  a  conducting  wire  sur- 
rounding it. 

Electricity  thus  produced  by  the  agency  of  a  magnet  is 
called  Magneto-electricity. 

This  may  be  shown  by  introducing  one  of  the 
poles  of  a  powerful  bar  magnet  within  a  helix  of 
fine  insulated  wire  (see  Fig.  370),  the  ends  of 
which  are  connected  with  a  delicate  galvan- 
ometer. The  deflection  of  the  needle  will  indi- 
cate the  flow  of  an  electric  current  every  time  the 
magnet  enters  or  leaves  the  coil — the  direction 
of  the  current  changing  with  the  poles  entered. 
The  same  results  will  be  obtained,  if  instead 
of  introducing  and  removing  a  permanent  steel 
magnet,  we  continually  change  the  polarity  of  a 
soft  iron  bar.  Thus,  in  Fig.  371,  let  a  &  be  a  bar 
of  soft  iron  surrounding  a  helix,  and  N  E  S  a 
horse-shoe  magnet  so  arranged  that  it  can  revolve 
freely  on  a  pivot  at  c,  the  poles  in  their  revolution  just  passing  by  the  termina- 
tions of  the  bar  a  b.  On  causing  the  magnet  to  re- 
volva,  the  polarity  of  the  bar  a  b  will  be  reversed 
with  every  half  revolution  the  magnet  makes,  and 
this  reversal  of  polarity  will  generate  electric  cur- 
rents in  the  wire. 

To    instruments    constructed    ©n 
these  principles  the  name  of  mag- 
neto-electric machines  is  given. 
can  one  eiec-          817.    Whenever    an 
duL'aSor?'    electric     current     flows 

througha  wire  it  excites  another  current  in  an 
opposite  direction,  in  a  second  wire  held  near  to  and 
parallel  with  it.  Its  duration,  however,  is  only  momentary. 
On  stopping  the  primary  current,  induction  again  takes 


440  WELLS'S   NATUKAL   PHILOSOPHY. 

place  in  the  secondary  wire  ;  but  the  current  now  arising 
has  the  same  direction  as  the  primary  one. 

The  passage  of  an  electrical  current,  therefore,  develops  other  currents  in 
its  neighborhood,  which  flow  in  the  opposite  direction  as  the  primary  one 
first  acts,  but  in  the  same  direction  as  it  ceases.  Whenever  a  magnet,  also, 
is  moved  in  the  neighborhood  of  a  conducting  wire,  these  currents  are  also 
induced. 

What  is  tin  818'  Magnet°-electr'lc  machines,  arranged  for  developing 
general  cou-  electricity  by  the  reaction  of  a  magnet,  are  constructed  in  a 
niagneto-dec?f  &reat  varietv  of  forms.  In  some,  permanent  steel  magnets  are 
trie  machines?  used ;  in  others,  temporary  soft  iron  ones,  brought  into  ac- 
tivity by  a  galvanic  current.  A  common  form  of  magneto- 
electric  machine  is  represented  in  Fig.  372. 

FIG.  372.  -ft  consists  of  a  compound  horse- 

shoe magnet,  S,  Fig.  372,  bolted 
to  a  mahogany  stand,  arranged  in 
such  a  manner  that  an  electro- 
magnet, or  armature,  A  B,  mount- 
ed on  an  axis,  revolves  in  front  of 
its  poles,  by  turning  a  multiplying 
wheel,  "W.  This  electro-magnet, 
or  armature,  consists  of  two  cores 
of  soft  iron  wound  about  with  fine 
insulated  copper  wire.  The  ends 
of  the  Avire  in  these  coils  are  kept 
pressed,  by  means  of  springs, 
against  a  good  conducting  metal  plato,  which  in  turn  is  connected  by  wires 
with  the  screw-caps  at  the  end  of  the  base  hoard!  When  the  iron  cores,  or 
axes  of  the  coils  are  in  front  of  the  poles  of  the  magnet,  they  become  mag- 
netic by  induction.  This  sets  in  motion  the  natural  electricity  of  the  coil,  or 
helices,  which  flows  in  a  certain  direction,  and  is  conveyed  through  the 
springs  and  wires  to  the  screw-caps. 

If  the  armature  be  turned  half  round,  the  magnetism  of  the  iron  is  reversed, 
and  a  second  current  is  excited  in  the  opposite  direction. 
What  effects  ^  turning  The  armature  very  rapidly,  a  constant  current 
may  be  pro-  passes  through  the  wires,  and  by  connecting  a  small  piece  of 
action  oTetoe!  Platinum  wire  in  the  circuit,  it  is  rapidly  rendered  red  hot. 
tro-magnelic  By  conveying  connecting  wires  from  the  magneto-electric 
machine  into  acidulated  water,  its  decomposition  is  effected; 
and  many  chemical  compounds  may  in  like  manner  be  resolved  into  their 
ultimate  constituents:  machines  also  of  this  character  maybe  used  fpr'electro- 
plating. 

The  effects  of  electricity  thus  generated  on  the  human  system  are  peculiar. 
If  the  two  handles  connected  with  the  screw-caps  of  the  machine  are  grasped 
by  the  hands,  slightly  moistened,  and  the  armature  is  made  to  revolve  rap- 


ELECTRO-MAGNETISM.  441 

idly,  the  muscles  are  closed  so  firmly,  that  the  handles  can  not  be  dropped, 
and  most  powerful  convulsive  shocks  are  sent  through  the  arms  and  body. 

what  isadia-  819.  It  has  been  demonstrated  by  Professor 
magnetic  bodj?  Faracjav  that  bodies,  not  in  themselves  mag- 
netic, may,  when  placed  under  certain  physical  conditions, 
be  repelled  by  sufficiently  powerful  electro-magnets.  Such 
substances  have  been  termed  diamagnetic,  and  the  phe- 
nomena developed  have  received  the  general  name  of  dia- 
magnetism. 

Bodies  that  are  magnetic  are  attracted  by  the  poles  of 
a  magnet  ;  bodies  that  are  diamagnetic  are  repelled  by 
the  poles  of  a  magnet.  Magnetism  may  be  regarded  as 
an  attractive  force,  diamagnetism  as  a  repelling  one. 

Thus,  if  a  bar  of  iron  is  suspended  free  to  move  in  FlG  ^3 

any  direction,  between  the  poles,  N  S,  of  a  magnet,      w 
Fig.  373,  the  bar  will  arrange   itself  along  a  line 
which  will  unite  the  two  poles  ;  it  places  itself  in  the 
axial  line,  or  along  the  line  of  force.     Such  is  the  con- 
dition of  a  magnetic  body.     If  a  substance  of  the  diamagnetic  class  is  placed 


FIG  374.  *n  tbe  same  8ituation  —  as>  for  examPle»  a  bar  °f  bis" 

muth  —  between  the  poles,  N  S,  Fig.  374,  it  places  it- 
self across,  or  at  right  angles  to  the  axial  line,  or  the 
line  of  force. 

Every  substance  in  nature  is  in  one  or  the  other  of 
these  conditions.  "  It  is  a  curious  sight,"  says  Dr. 
Faraday,  "  to  see  a  piece  of  wood,  or  of  beef,  or  an  apple,  or  a  bottle  of  water 
repelled  by  a  magnet  ;  or  taking  the  leaf  of  a  tree,  and  hanging  it  up  between 
the  poles,  to  observe  it  taking  an  equatorial  position." 


10* 


INDEX. 


,  spherical,  what  is,  329 
Abutment,  what  is  an,  120 
Acoustics,  188 

Acoustic  figures,  what  are,  189 
Actinism,  what  is,  843,  344 
Action,  and  Reaction,  66 

illustrations  of,  66 
laws  of,  66 

Action,  voltaic,  how    interrupted  and  re- 
newed, 40J 
Adhesion  defined,  29 
what  is,  25 

Aeriform  bodies,  how  exert  pressure,  174 
Aerolites,  constitution  of,  289 

what  are,  283 
Affinity  defined,  25 
Aim,  philosophy  of  taking,  295 
Air,  compressibility  of,  164 

capacity  of,  for  moisture,  268 

constituents  of,  163 

density  of,  165 

elasticity  of,  165      - 

fresh,  how  much  required  for  a  healthy 

man,  261 

heated,  why  rises,  261 
how  heated,  218 
in  spring,  why  chilly,  246 
in  water,  180 
inertia  of,  164 
momentum  of,  1ST 

illustrations  of,  1ST 
not  necessary   for  the  production  of 

sound,  191 
weight  of,  168 
when  rarefied,  166 
•when  said  to  be  saturated,  268 
why  unwholesome  after  having  been 

respired,  260 

pump,  construction  of,  176, 177 
Alphabet,  telegraphic,  435 
Anemometer,  '282 
Angle,  defined,  71 

of  incidence  and  reflection,  71 
Animals  foretell  changes  in  weather,  292 
Annealing  described,  '27 
Aqueducts,  construction  of,  134 
Arch,  base  of,  120 

springing  of,  120 
strength  of,  120 
what  is  an,  120 

why  stronger  than  a  horizontal  struct- 
ure, 120 


Archimedes,  experiment  with  the  crown,  44 

screw  of,  159 
Architrave,  121 
Architecture,  119 

orders  in,  120 

origin  of  different  styles  of, 

U9 

Armature  of  a  magnet,  423 
Artillery,  effective  distance  of,  77 
Artesian  wells,  135 
Astatic  needle,  what  is  an,  430 
Atmosphere,  composition  of,  163 

effect  of,  on  diffusion  of  light, 

302 

how  heated,  226 
pressure  of,  168 
supposed  height  of,  173 
what  is,  163 
Atmospheric  electricity,  391 

pressure,  effects  of,  174, 175 
how  sustained,  179 
refraction,  314 
Atom,  what  is  an,  13 

Attraction,  at  insensible  distances  illustrat- 
ed, 22 
cohesive,  25 

how  varies,  25 
illustration  of  simple,  18 
molecular,  four  kinds  of,  24 
mutual,  illustrations  of,  30 
what  is,  17 
Aurora  borealis,  cause  of,  396 

no  influence  on  the  weather,  291 
Auroras,  not  local,  397 

peculiarities  rf,  397 
Avoirdupois  weight,  34 
Axis  of  a  body,  what  is  an,  82 


B 


Balusters,  121 

Balance,  ordinary,  described,  97 

torsion,  882 

when  indicates  false  weights, 
Ballast,  use  of,  in  vessels,  139 
Balls,  cannon,  velocity  of,  76 
Balloons,  varieties  of,  186 

what  are,  188 
Balloon,  why  arises,  43 
Barker's  mill,  157 
Barometer,  how  invented,  169-171 
how  constructed,  111 
aneroid,  172 


444 


INDEX. 


Barometer,  water,  172 
wheel,  171 
how  indicates  weather  changes, 

173 
how  used  for  measuring  heights, 

Batteries,  thermo-electric,  417 
Battery,  Daniell's,  407 
galvanic,  401 
Grove's  galvanic,  construction  of, 

40G 

imperfections  of,  407 
luminous  effects  of,  410 
Smee's  galvanic,  406 
sulphate  of  copper,  406 
trough,  described,  405 
Beam,  rectangular,  strength  of,  116 

bent  in   the  middle,  why  liable   to 

break,  119 

or  bar,  when  the  strongest,  115 
Bellows,  hydrostatic,  1^9 
Bells,  electrical,  385 
Belts,  motion  communicated  by,  101 
Billiards,  principles  of  the  game  of,  72 
Blanket,  utility  of  the  nap  of,  219 
Blower,  use  of,  263 

Boats,  life,  how  prevented  from  sinking,  147 
Bodies,  form  of,  how  dependent  on  heat,  228 
form  of,  how  changed  by  centrifu- 
gal force,  83 
falling,  laws  of,  55 

force  and  velocity  depend  on 

what,  54 
lighter  than  water,  specific  gravity, 

how  determined,  3D 
non-lumiuous,  when  rendered  vid- 

ble,  301 

when  heavy  and  light,  33 
when  transparent,  294 
when  luminous,  294 
when  appear  white,  301 
when  solid,  liqQid,  or'gaseous,  24 
when  float  in.  air,  185 
Body,  what  is  a,  11 

when  called  hot,  206 

size  of,  how  affects  its  strength,  115 

•when  stands  most  firmly,  50 

when  rolls,  and  when  slides  down  a 

Elope,  51 

where  will  have  no  weight,  33 
Boiling-point,  depends  on  what,  241 

influence  of  atmospheric  pres- 
sure on,  242 
Boiler-flue,  258 
Boilers,  steam,  how  constructed,  256 

essentials  of,  257 

locomotive,  how  constructed,  258 
Bones  of  men  and  animals,  why  cylindri- 
cal, US 

Boxes  of  a  pump,  181 
Breath,  why  visible  in  winter,  274 
Breathing,  mechanical  operation  of,  181 
Breezes,  land  and  sea,  284 
Bridge,  Britannia  tubular,  118,  119 
Brittlcness,  what  is,  27 
Bubble,  soap,  why  rises  in  the  air,  43 
Buckets  of  wheels,  156 
Building,  strength  of  a,  on  what  depends,  119 
Buildings,  how  warmed  and  ventilated,  260 
Buoyancy,  what  is,  138 
Burning-glasses,  209 


Caloric,  what  is,  206 

Camera  obscura,  347 

portable,  360 
Canals,  how  constructed,  137 

locks  in,  137 
Cannon  bursting  by  firing,  28 

varieties  of,  77 
Capillary  Attraction,  i:5,  142 

illustrated,  143 
Capstan,  construction  of,  100 
Car  axles,  why  liable  to  break,  28 
Carriage,  high,  liable  to  be  overturned,  50 
Cask,  tight,  liquids  will  not  flow  from,  179 
Catoptrics,  312 
Cellars,  cool  in  summer,  warm  in  winter, 

why,  220 
Center  of  gravity  in  irregular  bodies  how 

found,  48 
when  at  rest,  46 
in  what  three  ways  sup- 
ported, 47 

Centripetal  Force,  79 
Champagne,  why  sparkles,  181 
Charcoal  marks,  why  stick  to  a  wall,  25 

why  black,  301 
Chemistry,  definition  of,  9 
Children,  why  difficult  to  learn  to  walk,  52 
Chimney,  draught  of,  262 

how  constructed,  262 

how  quickens  ascent  of  hot  air, 

262 

Chimneys,  when  smoke,  262 
Chord  in  music,  196 
Chain-pump,  construction  of,  160 
Climate,  what  is,  2(37 
Circuit,  galvanic,  401 
Clock,  common,  described,  58 
water,  principle  of,  151 
Clocks,  why  go  faster  in  winter  than  in  sum- 
mer, 60 

Clothing,  when  warm  and  when  cool,  220 
Clouds,  average  height  of,  274 
cirrus,  275 
cumulus,  275 
how  differ  from  fog,  273 
how  formed,  274 
nimbus,  277 
strains,  277 
variety  of,  275 
what  are,  273 

why  appear  red  at  sunset,  337 
why  float  in  the  atmosphere,  274 
Coals,  mechanical  force  of,  251 
Coal,  equivalent  to  active  power  of  man,  251 
Cogs  on  wheels,  101 
Cohesion  defined,  25 
Cold,  greatest  artificial,  211 

natural,  211 
what  is,  206 

Color  and  music,  analogy  between,  338 
Color,  no  effect  on  radiation  of  heat,  223 

origin  of,  P-2G 
Colors,  complementary,  331 

dark,  absorb  any  heat,  225 

how  affect  their  relative  appearance, 

332  *.,';%; 

of  natural  objects  on  what  depend, 

330 
simple,  what  are,  329 


INDEX. 


445 


Column,  height  of,  how  measured,  121 

what  is  a,  121 
Compass,  mariner's,  424 

ordinary,  424 

when  discovered,  426 
Compressibility,  what  is,  16 
Concord  in  music,  196 
Condensation,  what  is,  238 
Conduction  of  heat,  216 
Convection  of  heat,  216 
Cordage,  strength  of,  on  what  depends,  113 
Cork,  why  floats  upon  water,  43 
Cornea,  what  is  the,  349 
Coulomb's  torsion  balance,  382 
Countries  destitute  of  rain,  279 
Coughing,  sound  of,  how  produced,  204 
Cranes,  what  are,  105 
Crank  defined,  110 
Cream,  why  rises  upon  milk,  147 
Crying,  what  is,  205 
Cupping,  operation  and  principle  of,  1T5 
Currents,  electric,  how  exert  their  influence, 

429 
Cylinders,  strength  of,  118 


Daguerreotypes,  how  formed,  345 

Dead  point  explained,  112 

Declination  of  needle,  426 

Density,  what  is,  15 

Derrick,  what  is  a,  105 

Dew,  circumstances  that  influence  the  pro- 
duction of,  271 
does  not  fall,  271 

phenomena  and  production  of,  270 
when  deposited  most  freely,  271 

Dew-drop,  why  globular,  30 

Dew-point,  270 

not  constant,  270 

Diamagnetic  phenomena,  441 

Diamagnetism,  441 

Dioptics,  318 

Direction,  line  of,  49 

Discord  in  music,  196 

Distillation,  242 

Divisibility,  13 

Dovetailing,  what  is,  117,  118 

Drainage,  principles  of,  152 

Draught  of  chimney,  262 

Dresses,  black,  optical  effect  of,  333 

Drops,  prescription  of  medicine  by,  unsafe, 

Ductility,  what  is,  26 

Dust,  how  we  free  our  clothes  of  by  agita- 
tion, 20 
Dynamometer  described,  89 


Ear,  construction  of,  201,  202 

Earth,  bodies  upon,  why  not  rush  together, 

30 

cause  of  present  form  of,  83 
centripetal  force  at  equator  of,  83 
how  proved  to  be  in  motion,  84 
the  physical  features  of,  how  affect 

winds,  282 
the  reservoir  of  electricity,  376 


Earth,  telegraphic  communication  through, 

437 

Earth's  attraction,  law  of,  32 
Ebullition,  what  is,  241 
Echo,  conditions  for  the  production  of,  198 

what  is,  197 
Echoes,  when  multiplied,  198 

where  most  frequent,  1S8 
Egg-shell,  application  of  the  principle  of  the 

arch  in,  120 

Elastic  bodies,  results  of  collision  of,  68 
Elasticity  defined,  22 
Eel,  electrical,  391 
Electric  attraction,  870 

currents,  how  exert  their  influence, 

429 

fluid  non -luminous,  387 
light,  410 
repulsion,  370 
shock,  383 

spark,  duration  of,  388 
Electrical  battery,  S84 
induction,  377 
machines,  378 

Electricity  a  source  of  heat,  212 
atmospheric,  391 
conductors  and    non-conductors 

of,  273 

Du  Fay's  theory  of,  271 
Franklin's  theory  of,  271 
effect  of  on  a  conductor,  386 
experiments   of   Franklin  with, 

392 
frictional,    distinctive    character 

of,  407 
galvanic,  how  excited,  401 

how  differs  from  ordi- 
nary, 300 

how  discovered,  398 
Quantity   of,   what  is, 

408 

theory  of,  402 
intensity   of,  what  is, 

408 

what  is,  398 
how  evolves  heat,  409 
how  excited,  369 
how  exerts  a  magnetic  force,  481 
influence  on  the  form  of  bodies, 

376 

kinds  of,  370 
magneto,  437,  488 
cf  vital  action,  391 
positive  and  negative,  272 
quantity  necessary  for  decompo- 
sition, 412 

real  character  of  unknown,  403 
secondary  currents,  437 
thermo,  what  is,  416 
velocity  of,  how  determined,  889 
what  is,  369 

where  resides  in  bodies,  876 
Electro-magnetism,  429 

magnets,  how  formed,  482 

what  are,  432 
Electrometer,  381 
Electro-metallurgy,  413 
Electrophorus,  380 
Electroscope,  381 
Electrotyping,  413 
Electrodes,  what  are,  413 


446 


INDEX. 


Elements,  simple,  11 

number  of,  11 
Elevations,  how  determined  by  the  boiling 

point  of  water,  242 
Embankments,  why  made  stronger  at  the 

bottom  than  at  the  top,  132 
Endpsmose,  what  is,  146 
Engine,  fire,  construction  of,  183 

steam,  251-254 
Engraving,  how  affected  by  electro-metal- 


lurgy, 415 
Entablatui 


ire,  divisions  of,  121 

what  is,  121 
Equilibrium  indifferent,  48 

law  of,  in  all  machines,  92 
stable,  43 
unstable,  43 
what  is,  46 

Equinoctial  storm,  291 
Evaporation,  238 

circumstances  influencing,  239 
influence  of  temperature  on, 

240 

Ixosmose,  what  is,  146 
Expansibility,  what  is,  16 

illustrations  of,  16 
Expansion  by  heat,  228 

how  measured,  233 
Eye,  347 

how  judges  of  size  and  distance,  354 
how  moved,  348 
optic  axis  of,  353 
structure  of  in  man,  348 


Facade  of  a  building,  121 

Far-sightedness,  cause  of,  352 

Feather  attracts  the  earth,  32 

Fibrous  substances  non-conductors  of  heat, 

219 

Filtration  defined,  19 
Fire,  what  is,  209 

Fishes,  structure  of  the  body  of,  154 
Flame,  what  is,  209 
Flexibility,  what  is,  26 
Flies,,  how  walk  upon  ceilings,  1T6 
Floating  bodies,  laws  of,  138 
Fluid,  electric,  403 
Fluids,  what  are,  24 
Fly-wheel,  use  of,  IT 
Focus,  what  is  a,  322 
Form  of  bodies  dependent  on  heat,  228 
Forcing-pump,  construction  of,  183 
Force  defined,  21 

accumulation  of,  8T 
internal,  22 
magnetic,  418 
molecular,  22 
real  nature  of,  21 
Forces,  great,  of  nature,  21 
electro-motive,  401 

Fountains,  ornamental,  principle   of  con- 
struction of,  135 
Friction,  112 

advantages  of,  113 
how  diminished,  112 
kinds  of,  112 
rolling,  112 
eliding,  112 


Friction,  heat  produced  by,  214 

Freezing  mixtures,  composition  of,  245 

Frieze  in  architecture,  121 

Frost,  origin  of,  272 

Fuel,  what  is,  265 

Fulcrum  defined,  93 

Furs,  why  used  for  clothing,  219 

Furnaces,  hot-air,  264 

how  constructed,  265 


Galvanism,  398 

Galvanic  action,  how  increased,  403 
battery,  401 

heating  effects  of,  408 
physiological    effects   of. 

411 

Galvanometer,  430 
Gamut,  the,  196 
Gas,  how  differs  from  a  liquid,  29 

what  is,  23 
Gases,  how  expand  by  heat,  232 

specific  gravity,  how  determined,  41 
Gaseous  bodies,  properties  of,  23 
Gasometers,  construction  of,  179 
Gears,  in  wheel  work,  101 
Glass,  opera,  365 
Glasses,  sun,  209 
Glottis,  what  is  the,  203 
Glue,  why  adhesive,  25 
Grain  weight,  origin  of,  34 

bearing  plants,  construction  of  the 

stems  of,  118 

Gravitation,  attraction  of,  how  varies,  30 
defined,  30 
terrestrial,  32 

Gravity,  action  of,  on  a  falling  body,  55 
center  of,  45 
specific,  37 
Green  wood,  unprofitable  to  burn,  266 
Grindstones,    how  broken    by    centrifugal 

force,  80 

Guage,  barometer,  259 
steam,  259 
rain,  277 
Gun,  essential  properties  of,  76 
Gunpowder,  effective  limit  of  the  force  of, 

force  of  76 
Gurgle  of  a  bottle  explained,  180 


Hail,  what  is,  280 

storms,  where  most  frequent,  281 

stones,  formation  of,  281 
Halos,  what  are,  336 
Hardness,  what  is,  26 
Hearing,  conditions  for  distinctness  in,  200 

range  of  human,  203 
Heat,  205 

how  diffuses  itself,  206 

how  measured,  206 

distinguishing  characteristic  of,  206 

nature  of,  207 

theory  of,  207,  208 

and  light,  relations  between,  208 

devoid  of  weight,  209 


INDEX. 


447 


Heat,  sources  of,  209 

influence  extends  how  far  into  the 
earth,  211 

of  chemical  action,  212 

greatest  artificial,  212 

derived  from  mechanical  action,  213 

latent,  218 

sensible,  213 

of  vital  action,  214 

of  friction,  214 

conductors  and  non-conductors  of,  216 

radiation  of,  216 

communication  of,  216 

conducting  power  of  bodies,  how  di 
minished,  218 

good  radiators  of,  222 

how  propagated,  223 

Telocity  of,  223 

how  reflected,  224 

rays  of,  what  is  meant  by,  224 

absorption  of,  225 

expansion  by,  228 

how    transmitted    through   different 
substances,  226 

effects  of,  227 

solar,  compound  nature  of,  227 

force  of  expansion  of,  229 

expansion   of,  practical   illustrations 
of,  229 

latent,  when  rendered  sensible,  246 

capacity  for,  247 

quantity  of,  different  in  all  bodies,  247 

specific,  247 

Helix,  construction  of,  432 
Horse  power  defined,  88 
Houses,  haunted,  explanation  of,  200 
Humidity,  absolute  and  relative,  268 
Hurricane,  what  is  a,  285 
Hurricanes,  where  most  frequent,  285 
space  traversed  by,  286 
velocity  of,  286 
Hydraulics,  148     . 
Hydraulic  engines,  cause  of  the  loss  of  power 

in,  158,  159 

ram,  construction  of,  161,  162 
Hydrometer,  what  is  a,  141 

uses  of,  141 
Hydrostatics,  123 

Hydrostatic  press,  construction  of,  126, 127 
Hydro-extractor,  80 
Hygrometer,  how  constructed,  269 


Ice,  origin  of  bubbles  in,  232 

heat  in,  206 

Images,  when  distorted  in  mirrors,  303 
Impenetrability,  12 

illustrations  of,  13 
Incidence,  angle  of,  71 
Inclined  plane  described,  105 

advantage  gained  by,  106 
Induction,  magnetic,  421 
Inelastic  bodies,  results  of  collision  of,  69 
Inertia,  what  is,  16 

examples  of,  17 
Inkstand,  pneumatic,  179 
Insects,  how  produce  sound,  206 
Insulation,  374 
Intensity  in  electricity,  what  is,  408 


Iron,  galvanized,  what  is,  415 
how  made  hot,  206 
how  rendered  magnetic  by  induction, 

421 

ships,  principle  of  flotation  of,  140 
soft,  how  magnetized,  421 
why  stronger  than  wood,  29 


Kaleidoscope,  construction  of,  307 
Key-note,  what  is,  201 


Lakes,  salt,  origin  of,  124 
Lamp-wick,  how  raises  oil,  145 
Lantern,  magic,  what  is,  367 
Larynx,  description  of,  203 
Laughing,  what  is,  205 
Law,  physical,  definition  of  a,  10 
Lens,  achromatic,  3^8 
axis  of,  321 
denned,  319 
focal  distance  of,  321 
Lenses,  varieties  of,  319 
Level,  spirit,  construction  of,  137 

what  is  a,  53 

Lever,  law  of  equilibrium  of  the,  94 
Levers,  arms  of,  93 

compound,  96 

disadvantages  of,  97 
kinds  of,  93 
what  are,  93 
Leydenjar,  382 
Light,  absorption  of,  300 
analysis  of,  325 
chief  sources  of,  294 
corpuscular  theory  of,  293 
divergence  of  rays  of,  296 
electric,  410 
good  reflectors  of,  301 
how  analyzed,  326 
how  propagated,  295 
how  refracted  by  the  atmosphere, 

314 

intensity  of,  how  varies,  297 
interference  of,  339 
moves  in  straight  lines,  295 
polarized,  341 
polarization  of,  342 
ray  of,  what  is,  295 
refraction  of,  812 
same  quantity  not  reflected  at  all 

angles,  305 

three  principles  contained  in,  344 
undulatory  theory  of,  293 
velocity  of,  298 

how  calculated,  299 
vibrations  of,  339 
waves  of,  339 
what  is,  292 

when  totally  reflected,  316 
white,  composition  of,  326 
Lightning,  identity  of  with  electricity,  892 
mechanical  effects  of,  396 
rods,  how  constructed,  394 
space  protected  by,  395 
when  dangerous,  395 


448 


INDEX. 


Lightning,  varieties  of,  393 

Line,  vertical,  53 

Liquefaction,  what  is,  23T 

Liquid  at  rest,  condition  of  the  surface  of  a, 

133 

pressure  of  a  column  of,  128 
what  is  a,  23 

Liquids,  boiling  point  how  changed,  242 
flowing  from  a  reservoir,  149 
have  uo  particular  form,  23 
heat  conducting  power  of,  217 
how  cooled,  222 

move  upon  each  other  without  fric- 
tion, 124 
pressure  of,  125 

illustrated,  125 
why  some  froth,  180 
specific  gravity  how  found,  39 
spheroidal  state  of,  240 
to  what  extent  expanded  by  heat, 

229 
transmit  pressure  in  all  directions, 

125 

when  do  they  wet  a  surface,  30 
Loadstone,  what  is  a,  416,  417 
Locks,  canal,  how  operated,  137 
Locomotive,  efficacy  depends  on  what,  29 
Looking-glasses,  how  formed,  303 


Machine,  what  la  a,  90 
Machines  diminish  force,  90 

do  not  produce  power,  90 

how  make  additions    to   human 

power,  91 

how  produce  economy  of  time,  94 
motion  in,  takes  place  when,  92 
simple,  93 
Machinery,  elements  of,  93 

general  advantage  of,  92 
magnetic,  433 

•when  caught  on  a  center,  112 
Magdeburg  hemispheres,  17T 
Magnet,  rotation  of  a,  431 

when  traverses,  419 
Magnets,  artificial,  418 

horse-shoe,  420 
native,  417 

power  of  artificial,  423 
what  are  poles  of,  418 
Magnetic  induction,  421 
meridian,  425 
phenomena,    how  accounted  for, 

423 

polarity,  419 
power  of  a  body,  where  resides, 

421 
Magnetism,  417 

animal,  what  is,  418 
electro,  429 

how  excited  by  electricity,  432 
how  induced  by  the  earth,  422 
why  not  available  for  propel- 
ling force,  433 

Magneto-electric  machines,  438 
Magnifying  glasses,  324 
Magnitude,  12 

center  of,  45 


Malleability,  what  is,  26 

examples  of,  26 

Man,  how  exerts  his  greatest  strength,  88 
estimated  strength  of,  88 

Mariotte's  laws,  what  are,  166,  167 

Matter,  cause  of  changes  in,  21 
definition  of,  11 
essential  properties  of,  12 
indestructible,  18 
not  infinitely  divisible,  13 
smallest  quantity  visible  to  the  eye, 

Materials,  strength  of,  115 

upon  what   depend, 

115 
Matting,  how  protects  objects  from  frost, 

272 

Mechanical  powers,  93 
Meniscus,  B'20 
Meridian,  magnetic,  425 

of  the  earth  defined,  36 
Metals,  union  of,  how  affects  durability,  415 
Meteors,  how  differ  from  shooting  stars,  289 
Meteorites,  what  are,  288 
Meteoric  bodies,  supposed  origin  of,  289 

phenomena,  288 
Meteorology,  266 
Microscope,  compound,  361 
solar,  368 
what  is  a,  360 

Microscopes,  varieties  of,  361 
Milk,  why  cream  rises  upon,  147 
Mirage,  315 
Mirror,  plane,  how  reflects  light,  303 

what  is  a,  D02 
Mirrors,  burning,  308 
concave,  308 
convex,  311 
parallel,  effect  of,  306 
varieties  of,  302 
Mississippi,  does  it  flow  up  hill,  152 

quantity  of  water  in,  152 
Mists  and  fogs,  how  occasioned,  273 
Moisture  in  air,  how  determined,  269 
Molecule  defined,  14 
Momentum,  how  calculated,  65 
what  is,  64 
illustrations  of,  64 
Monsoons,  theory  6f,  '283 
what  are,  283 

Moon,  influence  of  on  weather,  291 
Motion,  absolute  and  relative,  62 

accelerated  and  retarded,  63 
apparent,  affected  by  distance,  359 
circular,  illustrations  of,  78 
compound,  72 

illustrations,  72 

imparted  to  a  body  not  instantane- 
ously, 65 

perpetual,  in  machinery,  not  possi- 
ble, 91 
perpetual,  in  nature,  91 

example  of,  91 
reflected,  what  is,  71 
reversion  of  by  belting,  101 
rotary,  111 
rectilinear,  111 
simple,  illustrations  of,  73 
uniform  and  variable,  63 
what  is,  62 
when  imperceptible  to  the  eye,  869 


INDEX. 


449 


Mortise,  what  is  a,  113 

Mountains,  height  of,   how  determined  by 

the  barometer,  173 
Movements,  vibratory,  nature  of,  188 
Mud,  why  flies  from  wheel  of  carriage,  79 
Muscular  energy,  how  excited,  87 
Music,  scale  in,  196 

notes  in,  how  indicated,  196 
Musical  sounds,  194 


N 


Natural  Philosophy,  definition  of,  9 
Near-sightedness,  cause  of,  352 
Needle,  magnetic,  423 

dipping,  425 

diurnal  variation  of,  428 

magnetic,  directive  power  of,  how 
explained,  420 

variations,  cause  of,  428 
Notes,  musical,  when  in  unison,  195 
in  music,  how  indicated,  196 


Ocean,  depth  of,  123 

extent  of,  123 
Octave  in  music,  195 
Oersted's  discovery,  429 
Oils,  how  diminish  friction,  112 
Opaque  bodies,  294 
Optical  instruments,  360 
Optics,  medium  in,  312 

science  of,  292 


Paddles  of  a  steamboat,  when  most  effect- 
ive, 154 

Paper,  blotting,  why  absorbs  ink,  147 
Parabola  denned,  74 
Paradox,  hydrostatic,  126 
Pendulum,  center  of  oscillation  of  a,  59 

compensating,  60 

described,  35 

influence  of  length  011  vibration 
of,  5'J 

length  of  a,  seconds,  61 

times  of  vibration  of,  58 

compared,  58 

used  as  a  standard  for  measure, 

61 

Perspective,  what  is,  356 
Photometers,  construction  of,  298 
Physics,  definition  of,  10 
Pilaster,  what  is  a,  121 
Pile,  in  architecture,  120 

Zamboni's,  explained,  404 
Piles,  voltaic,  404 
Pipes,  rapidity  of  water  discharged  from,  150 

water,  requisite  strength  of,  134 
Pisa,  leaning  tower  of,  49 
Pitch,  or  tone,  195 
Pkmts,  vital  action  of,  215 
Platform  scales,  98 
Pliability,  what  is,  26 
Plumb-line,  S3 
Pneumatics,  163 
Polarity,  magnetic,  419 


Poles,  magnetic,  where  situated,  426 

of  galvanic  battery,  what  are,  402 
Pop-gun,  operation  of,  1(37 
Pores,,  defined,  14 

evidence  of  the  existence  of,  15 
Porosity  defined,  14 
Porter,  why  froths,  180 
Portico,  what  is  a,  121 
Power,  agents  of  in  nature,  87 

and  resistance  defined,  63 

and  weight  in  machinery  defined,  92 

expended  in  work,  how  ascertained, 
89 

mechanical  effect  of,  how  estimated, 
92 

moving,  effect  ef,  how  expressed,  89 

space  and  time,  how  exchanged  for, 

92 

Press,  hydrostatic,  127" 
Prism  defined,  318 
Projectile,  what  is,  74 
Projectiles,  laws  of,  74 

range  of,  75 
Propellers,  advantage  over  paddle-wheels, 

155 

construction  of,  155 

Pugilists,  blows  of,  when  most  severe,  69 
Pulley  defined,  102 

kinds  of,  102 

fixed,  described,  102 

movable,  103 

Pulleys,  advantage  of,  104 
Pump,  air,  177 

chain,  160 

common  suction,181 

when  invented,  160 

forcing,  construction  of,  183 

Vera's,  145 
Pyrometers,  233 


Quantity  in  electricity,  what  is,  408 


Kadiation  of  heat  proceeds  from  all  bodies, 

223 
Rain,  what  is,  277 

why  falls  in  drops,  277 
formation  of,  on  what  depends,  277 
guage,  277 

yearly  estimated  quantity  of,  279 
where  most  abundant,  278 
Rain-bow,  what  is  a,  333 
when  seen,  335 
why  semicircular,  335 
Ram,  hydraulic,  construction  and  operation 

of,  161,  162 
Range  in  gunnery,  75 

greatest,  when  attained,  75 
Rays  of  heat,  what  meant  by,  224 
Reflection,  angle  of,  71 
Reflector  of  heat,  best,  225 
Refraction,  index  of,  316 
double,  340 

how  accounted  for,  317 
Refrigerators,  construction  of,  221 
Regulators  of  steam-engines,  256 
Repulsion,  what  is,  22 


450 


INDEX. 


Repulsion,  and  attraction,  magnetic,  419 

Retina  of  the  eye,  348 

Ricochet  firing,  77 

Rifle,  Minie",  construction  of,  77 
how  sighted,  78 

Rivers,  why  rarely  straight,  86 
velocity  of,  152 
water  discharge  of,  152 

Roads,  inclination  of,  how  estimated,  106 
how  should  be  made,  106 

Rods,  discharging,  380 

Room,  how  best  ventilated,  264 

Booms  for  speaking,  how  constructed,  201 

Rope-dancing,  art  of,  52 


Safes,  fire-proof,  how  constructed,  221 
.Sandstones,  why  ill  adapted  for  architectural 

purposes,  123  . 

Saw-dust,  utility  of  in  preserving  ice,  220 
Scales,  hay  and  platform  described,  98 
Scarfing  and  interlocking  117 
Scissors,  a  variety  of  lever,  94 
Screw,  advantage  gained  by,  109 

applications  of,  109 

defined,  108 

endless,  110 

Hunter's,  110 

nut  of,  108 

of  Archimedes,  159 

thread  of,  109 

Screw-Propeller,  what  is  a,  155 
Sea,  proximity  to,  mitigates  cold,  268 
Shadow,  what  is  a,  296 
Shadows,  how  increase  and  diminish,  297 
Shell,  sea,  cause  of  the  sound  heard  in,  199 
Ships,  copper  sheathing  of,  how  protected, 

iron,  why  float,  140 
Shooting-stars,  how  accounted  for,  290 
Short-sightedness,  cause  of,  352 
Shot,  how  manufactured,  30 
Silver,  adulteration  of,  how  detected,  43 
Simmering,  what  is,  241 
Skull,  human,  combines  the  principle  of  the 

arch,  120 
Smoke,  why  rises  in  the  air,  43 

why  ascends  in  chimney,  261 
rings,  origin  of,  187 
Sneezing,  what  is,  205 
Snow  crystals,  280 

flake,  composition  of,  280 
how  formed,  280 
line  of  perpetual,  248 
protective  influence  of  against  cold,220 
what  is,  280 

Soft,  when  is  a  body,  26 
Solar  microscope,  3(53 
Solid,  what  is  a,  23 
Solids,  why  easily  lifted  in  water,  139 

specific  gravity,  how  determined,  39 
Solution,  what  is  a,  237 

how  differs  from  a  mixture,  237 
when  saturated,  237 
Sound,  conductors  of,  192 

how  decreases  in  intensity,  192 
how  propagated,  190 
interference  of  waves  of,  194 
loudness  of,  on  what  depends,  194 


Sound,  reflection  of,  197 
velocity  of,  193 
what  is,  188 
when  communicated  most  readily, 

191 

when  inaudible,  190 
Sounds,  musical,  194 

not  heard  alike  by  all,  203 

seem  louder  by  night  than  by  day, 

Spark,  electric,  388 

Speaking,  rooms  suitable  for,  201 

Specific  gravity,  37 

how  discovered,  44 
how  found,  38 
standard  for  estimating,  38 
practical  applications  of,  44 
Spectacles,  what  are,  360 
Spectrum,  solar,  326 
Springs,  intermitting,  185 

origin  of,  136 
Spy-glass,  what  is,  364 
Stability  of  bodies,  depends  on  what,  48 
Stairs  are  inclined  planes,  107 
Stars,  shooting,  289 

height  of,  289 
Steel,  how  tempered,  27 

how  magnetized,  421 
Steel-yard  described,  97 
Steam,  advantages  of  heating  by,  265 
elastic  force  of,  249 
superheated,  250 
high  pressure,  250 
formed  at  all  temperatures,  239 
guage,  259 

how  rendered  useful,  252 
pressure  of,  how  indicated,  260 
true,  invisible,  238 
when  used  expansively,  255 
Steam-boilers,  cause  of  explosion  of,  258 
whistle,  260 
engine,  what  is,  251 

condensing,  253 
construction  of,  253 
high  pressure,  254 
regulators  of,  255 
greatest  amount  of  work  per- 
formed by,  251 

Stethoscope,  construction  of,  192 
Still,  construction  of,  243 
Structure,   influence   of   the  parts  on  the 

strength  of  a,  117 

Stone  for  architectural  purposes,  how  se- 
lected, 122 
Stool,  insulating,  380 
Stove,  why  snaps  when  heated,  230 
Stoves,  how  differ  from  open  fire-place,  263 
disadvantages  of,  264 
why  placed  near  the  floor,  261 
Sublimation,  what  is,  243 
Sucker,  the  common,  175 
Suction,  what  is,  169 
Sugar,  how  refined,  242 

how  absorbs  water.  145 
Sun  does  not  really  rise  and  set,  84 
heat  of,  why  greatest  at  noon,  210 
the  greatest  natural  source  of  heat,  209 
nature  of  the  surface  of,  210 
Surface  defined,  12 

spherical,  definition  of  a,  133 
Syphon,  what  is  a,  184 


INDEX. 


451 


Syphon,  action  of,  184 
Syriuge,  principle  of,  176 


Tackle  and  fall,  what  is  a,  105 
Telegraph,  atmospheric,  181 
Bain's,  436 
chemical,  436 
House's,  436 
Morse's  magnetic,  434 
printing,  436 
Telegraphic  method,  the  first  proposed,  436 

wires,  insulation  of,  436 
Telescope,  equatorial,  363 
reflecting,  365 
refracting,  363 
what  is  a,  363 
Temperature,  average,  how  found,  267 

greatest  natural  ever  observ- 
ed, 210 

in  winter  and  summer,  differ- 
ence between,  210 
meaning  of,  206 
varies  with  latitude,  267 
Tenacity,  what  is,  26 
Theory,  physical,  definition  of  a,  10 
Thermometer,  233 

how  graduated,  234 
centigrade,  235 
mercurial,  described,  234 
Reaumur,  described,  235 
Thermometer-air,  described,  236 
Thermo-electricity,  what  is,  416 
Thunder,  cause  of,  393 

storms,  where  most  prevail,  394 
Tides,  origin  of,  32 

Toes,   advantage  of  turning  out  in  walk- 
ing, 50 

Tone  in  sound,  195 
Tongueing,  what  is,  117 
Tornadoes,  how  produced,  287 

what  are,  287 

Torpedo,  electrical  effects  of,  391 
Torricelli's  invention,  169,  170 
Trade  winds,  cause  of,  283 

direction  of,  283 
Transparent  bodies,  294 
Troy  weight,  34 
Trumpet,  ear,  what  is  an,  200 

speaking,  construction  of,  199 
Tubes,   capillary,   height    to  which   water 

rises  in,  143 
Twilight,  314 
Twinkling,  what  is,  338 
Tympanum  of  the  ear,  202 


Vacuum,  what  is  a,  168 
Valve,  definition  of,  182 

safety,  258 

Variation,  lines  of,  426 
Vapor,  always  present  in  air,  239 

appearance  of,  238 
Vapors,  elasticity  of,  248 

formed  at  all  temperatures,  238 
Vault,  what  IB  a,  120 
Velocity  defined,  63 


Velocity  of  moving  body,  how  determined, 

63 

Vena  contracta,  what  is  the,  150 
Ventilation,  what  is,  260 

when  perfect,  260 

Vessels  of  liquid,  pressure   upon  the  bot- 
tom of,  131 

Vibrations  of  sound,  nature  of,  188,  189 
Views,  dissolving,  368 
Vision,  angle  of,  354 

deceptions  of,  357 
double,  how  produced,  354 
phenomena  of,  347 
Vital  action,  214 
Voice,  compass  of,  200 

how  produced,  203 
organs  of,  201 
Voltaic  piles,  404 
Volume  defined,  12 


W 


Walls,  how  deafened,  192 
Warming  and  ventilation,  260 
Warp  and  woof,  117 
Watch,  how  differs  from  a  clock,  59 
Water  as  a  motive  power,  155 
boiled,  why  flat,  180 
boiling,  temperature  of,  241 
composition  of,  123 
compressibility  of,  124 
decomposition  of,  412 
elasticity  of,  124 
expands  in  freezing,  231 
force  of  expansion  of  in  freezing,  232 
freezing  temperature  of,  232 
greatest  capacity  of  all   bodies  for 

heat,  248 

how  high  rises  in  a  pump,  182 
how  made  hot,  221 
illustrations  of  the  pressure  of,  130 
imparts  no  additional  heat  after  boil- 
ing, 244 
inclination  sufficient  to  give  motion 

to,  152 
level,  136 
power  defined,  88 
pressure  at  different  depths,  131 

how  calculated,  132 
sound  of  falling,  how  produced,  204 
spouts,  what  are,  287 
supply  of  towns,  134 
to  what  degree  can  be  heated,  249 
velocity  of  in  pipes,  how  retarded,  151 
when  has  its  greatest  density,  231 
why  rises  by  suction,  169 
why  rises  in  a  pump,  182 
Waters,  comparative  purity  of,  123 
Wave,  a  form,  not  a  thing,  153 
Waves,  height  of,  153 

optical  delusions  of,  153 
origin  of,  153 
of  sound,  190 

Weather,  popular  opinions  concerning,  291 
Wedge,  what  is  a,  106 

when  used  in  the  arts,  107 
how  the  power  of  increases,  107 
Weight,  absolute,  what  is,  38 

how  determined  by  spe- 
cific gravity.  42 


452 


INDEX. 


Weight,  how  varies,  32 

of  a  body,  when  greatest,  32 
Weights  and  measures,  standards  of,  34 
French  system  of,  described,  36 
United  States  standard  of,  36 
"Welding  described,  29 
Well-sweep,  old  fashioned,  150 
Wells,  Artesian,  construction  of,  136 

origin  of  water  in,  136 
Wet  clothes,  why  injurious,  246 
Wheel  and  axle,  action,  of  99 
spinning,  101 

tourbine,  advantages  of,  158 
work,  compound,   familiar  illustra- 
tions of,  101 

Wheels,  breast,  construction  of,  157 
cog,  101 
overshot,  157 
tourbine,  158 
undershot,  156 
"  paddle,  power  lost  by,  154 
Whirlwinds,  how  produced,  267 
Whistle,  steam,  269 


Winch,  what  is  a,  99 

Wind,  principal  cause  of,  281 

what  is,  291 

Wind-pipe,  what  is  the,  203 
Windlass  described,  100 
Winds,  force  of,  how  calculated,  282 
of  United  States,  285 
trade,  283 
variable,  where  prevail,  284 

velocity  of,  281 
Wood,  a  bad  conductor  of  heat,  218 

comparative  value  of  for  fuel,  266 

hard,  why  difficult  to  ignite,  266 

made  wet,  why  swells,  148 

snapping  of,  19 

water  in,  265 

weight  of,  2G6 

Woods,  when  hard  and  when  soft,  265 
Wo  >lens,  why  used  for  clothing,  219 
Woof  of  cloth,  117 

Work  of  different  forces,  standard  of  com- 
paring, 88 
Working-point  in  machinery,  92 


A/ 


t 


<;# 


0983S 


;.-.  ft 


